SOUND AND THERMAL INSULATION

Abstract: A thermal insulating and/or sound absorbing structure comprising a batting of resilient, elongatable, nonlinear, nonflammable carbonaceous fibers, said fibers having a reversible deflection ratio of greater than 1.2:1, an aspect ratio greater than 10:1 and an LOI value greater than 40.

Description: SOUND AND THERMAL INSULATION The present invention relates to a nonflammable insulation material having a high degree of thermal insulation properties at a low bulk density and which also possesses excellent sound attenuating and dampening properties. More particularly, the invention is concerned with resilient, shape reforming, lightweight, nonflammable structures of carbonaceous materials having low heat conductivity, excellent thermal insulation and/or sound absorbing properties. The structures are further characterized by having good shape and volume retention that are stable to numerous compression and unloading cycles. Advanced thermal protection materials will have to meet demands for an acceptable environment. Smoke toxicity, outgassing, dust and other irritants, that may be generated or produced by the material are a problem not only for humans but also for equipment. Synthetic resinous thermal insulation materials that are commonly used are unacceptable because they are highly flammable and can generate copious quantities of toxic fumes. For applications in vehicles such as commercial aircraft,' spacecraft, satellites, and the like, smoke generation or outgassing can be fatal to- passengers and may also contaminate sensitive instruments, optical surfaces and the like, or, react chemically with various operative components of the vehicle. These pollutants can be controlled in part by the selection of fibers, 10 coatings, and proper pre- or post-treatments of flammable materials to minimize outgassing. Most applications for aircraft, for example, require reduced amounts of volatile materials in the aircraft. Highly _,(- crystalline, fully cross-linked or thermosetting polymeric materials have been used that exhibit a relatively inert behavior when subjected to a flame. However, such materials are still flammable!, 0 The prior art has used various materials, such as asbestos, glass wool, mineral wool, polyester and polypropylene fibers, carbon and graphite felts made from relatively short and straight or linear fibers, fowl down and various synthetic resinous foam materials 5 such as polyurethane foam, as thermal insulation for many applications. While asbestos, carbon and graphite felts, glass fiber and mineral wool are considered nonflammable, the other aforementioned thermal insulating materials are considered flammable. The bulk densities of some of the well-known thermal insulating materials are in the range of from 5.6 to 32 kg/n for insulating materials useful at temperatures not exceeding 120°C to densities of from 32 to 80 kg/m^ c for high temperature insulating materials. Even the newest "lightweight" insulating materials recently disclosed by NASA consisting of a ceramic material from which a carbonaceous material has been burned out has a bulk density of from 32 to 96 kg/m^. In addition, many of the lightweight thermal insulation materials which are made of a blend of spun and drawn, crimped, staple, synthetic polymeric fibers having a diameter of from 3 to 12 microns and synthetic polymeric staple fibers having a diameter of more than 12 and up to 50 microns are not fireproof and do not provide good sound 0 absorbing properties. U.S. Patent No. 4,167,604 (William E. Aldrich) discloses the use of crimped hollow polyester filaments in a blend with down (fowl) in the form of a multiple 5 ply carded web which is treated with a ther osetting resin to form a bat having thermal insulating characteristics. The web, however, is not flame retardant and is not a good sound absorbent. 0 U.S. Patent No. 4,321,154 (Francois Ledru) relates to a high temperature thermal insulation material comprising mineral fibers and pyrolytic carbon. To make the insulation lightweight, an c expanding agent or hollow particles such as microspheres are utilized. U.S. Patent No. 4,193,252 (Shepherd, et al) discloses the preparation of partially carbonized, graphite and carbon fibers from rayon which has been knitted into a fabric. When the fabric is deknitted, • the partially carbonized and the carbonized fibers contain kinks. .The fully carbonized or graphite fibers have kinks which are more permanent in nature. Applicants have found that partially carbonized rayon fibers do not retain their reversible deflection and -It- lose their kinks at relatively low temperatures or under tension. The fully carbonized or graphite yarn is brittle and difficult, if not impossible, to handle when deknitting. Moreover, carbon fibers produced from rayon are known to possess high water absorption and lower thermal conductivity than fibers with a higher graphite content, such as fibers prepared from an acrylic composition. In accordance with ' the present invention there is provided a lightweight, nonflammable structure composed of a multiplicity of nonlinear carbonaceous fibers which possess both excellent thermal insulation and/or sound absorbing properties. More particularly, the present invention is concerned with a structure comprising a multiplicity of resilient carbonaceous or carbon fibers having a sinusoidal or coil-like shape, a reversible deflection ratio of at least about 1.2:1, an aspect ratio (1/d) of greater than 10:1 and a limited oxygen index value of greater than 40. The structure has a bulk density of from 2.4 to 32 kg/ux- , preferably from 2.4 to 8 kg/π , for lightweight structures ' that are used for clothing articles, for example, and from greater than 8 kg/m^ to 32 kg/m^ for higher weight structures that are used for furnace insulation, for example. . The present invention Is specifically concerned with structures comprising a multiplicity of non¬ flammable, nonlinear, carbonaceous or carbon filaments containing at least 65 percent carbon. The filaments are particularly identified by their degree of carbonization and/or their degree of electrical conductivity in the determination of the particular use for which they are most suited. In accordance with one embodiment of the invention, the nonlinear, carbonaceous filaments which are utilized in the thermal insulating and/or sound absorbing structures of the invention are non- electrically conductive filaments which are formed by the partial carbonization of stabilized acrylic fibers or fabric or some other stabilized carbon fiber precursor material under conditions to impart a sinusoidal and/or a coil-like configuration to the fibers. The filaments are further characterized by their wool-like, fluffy appearance and texture when formed into nonwoven mats or batting. As will become apparent, the greater the amount of the filaments present in 'the structure, the greater will be the wool¬ like texture and resiliency. The fibers may be blended with noncarbonaceous fibers or carbonaceous linear fibers. The term nonconductive as utilized in the present application relates to a resistance of greater than 4 x 10° ohms/cm on a 6 tow formed from fibers having a diameter of from 7 to 20 microns, preferably 7 to 12 microns. When the precursor fiber is an acrylic fiber, it has been found that a nitrogen content of greater than 18.8 percent results in a nonconductive fiber. In accordance with a second embodiment of the invention, the nonlinear, carbonaceous filaments which are utilized in the structures of the invention comprise carbonaceous filaments having a low degree of electrical conductivity and a carbon content of less than 85 percent. Preferably,the carbonaceou's fibers are derived from oxidized acrylic fibers and possess a percent nitrogen content of from 10 to 35 percent, most preferably from 20 to 25 percent. The larger the amount of carbon content of the fibers utilized, the higher the degree of electrical conductivity. These high carbon filaments still retain a wool-like appearance when formed into a mat or a batting, especially when the majority of the fibers are coil¬ like. Also, the greater the percentage of coil-like fibers in the structure, the greater the resiliency of the structure. As a result of the greater carbon content, the structures prepared with the filaments of the Invention have greater sound absorbing properties and result in a more effective thermal barrier at higher temperatures. Lo 'conductivity means that a 6K tow of fibers has a resistance of fr^m 4 x 10° to 4 x 1θ3 ohms/cm. In accordance with a third embodiment of the invention, the nonlinear, carbonaceous or carbon filaments which are utilized in the thermal insulating and/or sound absorbing structures of the invention have a carbon content of at least 85 percent. Preferably, the filaments which are utilized are derived from stabilized acrylic fibers and have a nitrogen content of less than 10 percent. As a result of the higher carbon content, the structures are more electrically conductive. That is, the resistance of a 6K tow of fibers is less than 4 x 10^ ohms/cm. These fibers can be utilized in place of conventional straight or linear carbon fibers. Moreover, the coil-like carbonaceous or carbon filaments when formed into a structure such as a mat or batting, surprisingly provide better insulation against high heat and sound than an equal weight of linear- carbon fibers. A structure containing the greater amount of the coil-like fibers as compared to sinusoidal or linear fibers, provides a more effective barrier against heat and sound. Figure 1 is a graph of the heat insulating properties of a wool-like fluff of the invention as an insulation .material for furnaces. The thermal insulating and/or sound absorbing structures of the invention comprise a batting formed from nonlinear, nonflammable, resilient, elongatable carbonaceous fibers having a reversible deflection ratio of greater than 1.2:1, an aspect ratio (1/d) of greater than 10:1 and a limited oxygen index value of greater than 40. The carbonaceous fibers may possess a sinusoidal or a coil-like configuration or a more complicated, i.e. convoluted, structural combination of the two. The fibers of the invention when subjected according to the test method of ASTM D 2863-77 have a LOI value greater than 40. The test method is also known as "oxygen index" or "limited oxygen index" (LOI). With this procedure the concentration of oxygen in 0 2 /N mixtures is determined at which a vertically mounted specimen is ignited at its upper end and just (barely) continues to burn. The width of the specimen is from 0.65 to 0.3 cm with a length of from 7 to 15 cm. The LOI value is calculated according to the equation: [0 LOI = x 100 [0 + [N 10 The LOI value of a number of fibers is as follows: polypropylene . 17.4 polyethylene 17.4 15 polystyrene " 18.1 rayon 18.6 cotton 20.1 nylon • 20.1 polycarbonate 22 20 rigid polyvinyl chloride 40 oxidized polyacrylonitrile > 40 graphite -55 Such carbonaceous fibers are prepared by heat 25 treating a suitable stabilized precursor material such as that derived from an assembly of stabilized polyacrylonitrile based fibers, pitch based (petroleum or coal tar) fibers or fibers made from other polymeric 30 materials which can be made into nonlinear fiber or filament structures or configurations and which are thermally stable. For example, in the case of polyacrylonitrile - - (PAN) based fibers, the fibers are formed by melt or wet spinning a suitable fluid composition of the precursor material having a normal nominal diameter of from 4 to 25 micrometers, The fibers are collected as an assembly of a multiplicity of continuous filaments in tows and are stabilized (by oxidation in the case of PAN based fibers) in the conventional manner. The oxidation stabilized fiber tows (or staple yarn made from chopped or stretch broken fiber staple) are thereafter, " in accordance with the present invention, formed into a coil-like and/or sinusoidal form by knitting the tow or yarn into a fabric or cloth (recognizing that other fabric forming and coil forming methods can be employed). The so-formed knitted fabric or cloth is thereafter heat treated, in a relaxed and unstressed condition, at a temperature of from 525 to 750°C, in an inert atmosphere for a period of time sufficient to produce a heat induced thermoset reaction wherein additional cross-linking and/or a cross-chain σyclization reaction occurs between the original polymer chain. At the lower temperature range of from 150 to 525°C, the fibers are provided with a varying proportion of a temporary to a permanent "set," while in the upper range of temperatures of from 525 C C and above, the fibers are provided with a permanent "set." What is meant by "permanent set" is that a fiber possesses a degree of resiliency which manifests itself in a "reversible deflection" of the fiber when it is placed under stress such that the fiber is substantially linear in shape and where, upon release of the stress, the fiber returns to its unstressed condition. It is, of course, to be understood that the fibers or fiber assembly may be initially heat treated at the higher range of temperatures so long as the heat treatment is conducted while the coil-like and/or sinusoidal configuration is in a relaxed or unstressed state and under an inert, nonoxidlzing atmosphere. As a result of the higher temperature treatment, a permanently set, coil-l,ike or sinusoidal configuration or structure is imparted to the fibers in a yarn, tow or thread. The resulting fiber, tow or yarn having the nonlinear structural configuration (which are derived by deknitting the cloth) are subjected to other methods of treatment known in the art to create an opening, a procedure in which the fibers, yarn or tow of the cloth are " separated into a nonlinear, entangled, wool-like fluffy material in which the individual fibers retain their coil-like or sinusoidal configuration yielding a fluff or batting-like body of considerable loft. The fluff or batting of the invention may be utilized alone or may be provided with a suitable barrier layer of flexible sheet material or metal depending upon its desired use. The stabilized fibers when permanently set in accordance with the present invention into the desired structural configuration, e.g., by knitting, and thereafter heating at a temperature of greater than about 550°C, retain their resilient and reversible deflection characteristics. It is to be understood that higher temperatures may be employed of up to about 1500°C. However, the most flexible and smallest loss of fiber breakage, when carded to produce the wool-like fluff, is found in those fibers and/or filaments which are heat treated to a temperature " of from 525 to 750°C. The carbonaceous material which is utilized in the thermal insulating and sound absorbing structures of the invention may be classified into three groups depending upon the particular use and the environment that the structures in which they are incorporated are .placed. In a first group, the nonflammable, nonlinear j . carbonaceous fibers are electrically nonconductive and the fibrous batting may be used in connection with, for example, clothing or sleeping blankets because of its excellent washability and low moisture absorbency. The fibers . may be blended with other synthetic or natural 10 fibers including cotton, wool, polyester, polyolefin, nylon, rayon and the like. In order to test the moisture absorbent characteristics of the wool-like fluff of the 15 invention, several tests were conducted with batting having a density of from 2.4 to 8 kg/m^. Samples of the carbonaceous fiber batting were submerged in water and removed and wrung out by hand until no dripping of water was observed. The moisture retention of the 0 samples, based on the total weight of the batting and the moisture remaining in the batting was from 10 weight percent to less than 20 weight percent. With such a small amount of moisture remaining in the ~ batting, the batting still retains its thermal insulation characteristics. In a second group, the nonflammable, nonlinear carbonaceous fibers are classified ' as being partially 0 electrically conductive (i.e., having low conductivity) and have a carbon content of less than 85 percent. When the precursor stabilized fiber is an acrylic fiber, i.e., a polyacrylonitrile based fiber, the percentage nitrogen content is from 10 to 35 percent, 5 preferably, from 20 to 25 percent. These particular fibers in the form of a wool-like fluff, are excellent for use as insulation for vehicles, in particular commercial or military aircraft where light weight, nonflammable and sound absorbing characteristics in insulation structures are highly desirable. Such insulating structures are also highly desirable as insulation in areas where public safety is a major concern, such as in public buildings, for example hospitals, hotels, office buildings and the like. The structures formed from the wool-like fluff are also extremely light in weight, are nonflammable, have low moisture absorbency, good abrasive strength, good appearance and handle. In a third group are fibers having a carbon content of at ' least 85 percent. These fibers, as a result of their high carbon content, have superior thermal insulating and sound absorbing characteristics. The coil-like structure.in the form of a wool-like fluff (when carded) provides an Insulation which has good compressibility and resiliency while maintaining improved thermal insulating efficiency. The structure prepared with the these fibers has particular utility as high temperature insulation of, for example, furnaces and in various other areas of high heat and noise. The carbonaceous precursor materials which are advantageously utilized in preparing the stabilized fibers and wool-lilce fluff of the invention are selected from acrylonitrile homopolymers, acrylonitrile copolymers (in particular PANOX or GRAFIL-01) and acrylonitrile terpolymers. The copolymers preferably contain at least about 85 mole percent of acrylonitrile units and up to 15 mole percent of one or more mono- vinyl units copoly erized with styrene, methylacrylate, methyl methacrylate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the like. Also, the acrylic filaments may comprise terpolymers, preferably, wherein the acrylonitrile units are at least about 85 5 mole percent. It is to be further understood that carbonaceous precursor materials may have imparted to them an electrically conductive property on the order 10 of that of metallic conductors by heating the fibers, wool-like fluff or the batting-like shaped material to a temperature above 1000°C in a nonoxidizing atmosphere. The electroconductive property may be obtained from selected additional starting materials such as pitch 15 (petroleum or coal tar), polyacetylene, polyphenylene, polyvinylidene chloride resin (SARAN, a Trademark of The Dow Chemical Company) and the like. Preferred precursor materials are prepared by 20 melt spinning or wet spinning the precursor materials in a known manner to yield a monofilament fiber, filament yarn, or tow. A woven or knitted cloth can be produced from the fiber, yarn, or tow by any of a pt - number of commercially available techniques. The stabilized material is then heated to a temperature above 525°C, preferably to a temperature above 550°C in an inert atmosphere. The carbonized knitted cloth is thereafter deknitted and carded to produce the wool- 0 like fluff which can be laid up in batting-like form. The wool-like fluff of the invention may be treated with an organic or inorganic binder, needle punched, bagged or adhered to a flexible or rigid 5 support using any of the conventional materials and techniques depending upon the use and environment of the structure. The fluff may -be placed on one side of a structure such as a furnace or between structural parts either in.the form of a mat or batting. It is understood that all percentages as herein utilized are based on weight percent. Exemplary of the present Invention are set forth in the following examples: Example 1 A stabilized polyacrylonitrile PANOX (R.K. Textiles) continuous 3K or 6K (3000 or 6000 fiber) tow, hereafter referred to as 0PF, having nominal single fiber diameters of 12 micrometer, was knit on a flat bed knitting machine into a cloth having from 3 to 4 loops/cm. Portions of this cloth were heat set at one of the temperatures se forth in Table I, over a 6 hour period. When the cloth was deknitted, it produced a tow which had an elongation or reversible deflection ratio of greater than 2:1. The deknitted tow was cut into various lengths of from 5 to 25 cm, and fed into and opened by a Platts Shirley Analyzer. The fibers of the cut tow were separated by a carding treatment into a wool-like fluff, that is, the resulting product resembled an entangled wool-like mass or fluff in which the fibers had a high interstitial spacing and a high degree of interlocking as a result of the coiled and spring-like configuration of the fibers. The fiber lengths of each such treatment were measured and the results of these measurements set forth in Table I, following. TABLE I Fiber Staple Heat Length Treatment Stitches/ Run # (cm) in °C cm Tow Size 1 15. 550 4 3K 2 5 550 4 3K 3 10 650 3 6K 4 10 950. 3 6K 5 20 750 3 6K 6 25 950 4 6K Range of Fiber Length of Majority Run # Lengths (cm) of Fibers (cm) 1 3.8 - 15 13 - 15 2 2. .5 - 5 2.5 ' - 5 3 5. ,0 - 10 7.5 - 10 1 4 3. 8 - 9.5 7.5 - 9.5 5 7. 5 - 19 15.0 - 19 6 7. 5 - 23 19.0 - 23 The aspect ratio of each of the fibers was greater than 10:1 and each possessed a LOI value of greater than 40. Example 2 A series of runs ware made to determine the effect various heat treatment temperatures had on the fibers. A significant property was the specific resistivity of the fibers. To determine such property, numerous samples of an oxidation stabilized polyacrylonitrile yarn (PAN0X manufactured by R.K. Textiles of Heaton-Noris, Stockport, England), having a density from 1.35 to 1.39 g/cc and having either 3K or 6K filaments per tow, respectively was knitted into a plain jersey flat stock having from 3 to 4 stitches/cm, respectively. The cloth was placed under an oxygen- free nitrogen pad in an incremental quartz-tube • furnace. The temperature of the furnace was gradually increased from room temperature to about 750°C over a three-hour period with the higher temperatures being achieved by 50°C increments every 10 to 15 minutes. The material was held at the desired temperature for about one hour,- the furnace opened and allowed to cool while purging with argon. The specific resistivity of the fibers was calculated from measurements made on each sample using a measured average of six measurements, one made from fibers removed at each corner of the sample and one made from fibers removed from each edge, approximately at the middle of .the sample. The results are set forth in Table II, following. TABLE II Log Specific Percent Resistivity Final Temp. Weight Measured in in °C Loss ohm/cm 500 - 4.849 550 33 600 2.010 650 34 750 37 -1.21 850 38 -2.02 900 42 -2.54 995500 4455 .. -2.84 1000 48 -3.026 1800 51 -3.295 All of ' the above fibers had an LOI greater than 40 and an aspect ratio greater than 10:1. The analysis of the heat treated fibers was as shown in Table III, following. TABLE III Temperature (°C) C %U %E ambient (0PF) 58.1 19.6 3.8 450 66.8 19.4 2.2 550 69.9 18.9 1.9 650 69.7 18.1 1.6 750 73.0 17.8 1.1 Example 3 A fabric was knitted from a 3K or 6K PANOX OPF (R.K. Textiles) continuous, stabilized, filament tow on a Singer flat bed knitting machine and heat treated at temperatures until thermoset. The fabric was then deknitted and the spring-like, coil shaped tow fed directly into a carding machine. The resulting wool¬ like fluff was collected onto a rotating drum and had sufficient integrity to enable it to be easily handled. The fibers treated at a temperature of 550°C and made into a wool-like fluff are particularly suitable as insulation for clothing such as parkas, sleeping blankets, and the like, because of its excellent hand. The fluff can also be used to insulate structures for sound and against extreme temperature. Fibers treated at a slightly higher temperature of from 550°C to 650°C and made into fluff are particularly suitable for use as Insulation in, for example, vehicles, particularly airplanes. A wool-like fluff treated at a temperature of 950°C was highly electrically conductive and had a resistance of less than 75 ohms at any probe length taken at widely separated distances (up to 60 cm) in the wool-like fluff. The fibers were suitable for use as insulation around, for example, engines or turbines to absorb noise. The results of the examples are summarized in Table IV, following. TABLE IV Fiber Staple Heat Treatment Run # Length (cm) °C Stitches/cm 1 7.5 550 4 2 10 650 3 3 15 ' 650 3 4 20 950 3 5 25 950 3 Range of Fibers Run # Tow Size Lengths (cm) 1 3K 2.5 - 7.5 2 2 6 6KK 2.5 - 10 3 6K 2.5 - 13.3 4 6k 2 - 15.0 5 6K 2 - 12.5 *• The experiments also illustrate that the higher temperature heating resulted in a shrinkage of the fibers. Example 4 A 3K 0PF PAN0X stabilized tow was knit on a Singer flat bed knitting machine at a rate of 4 stitches/cm and was then heat treated at a temperature of 950°C. The cloth was deknitted and the tow (which had a coil elongation or reversible deflection ratio of greater than 2:1) was cut into 7.5 cm lengths. The cut tow was then carded on a Platt Miniature carding machine to produce a wool-like fluff having fibers ranging from 2.5 to 6.5 cm ia length. The wool-like fluff had a high electrical conductivity (a resistance of less than 4 x 10^ ohms/cm) over any length of up to 60 cm tested. Instead of PANOX, there may be employed c stabilized pitch based fibers or a copolymer or terpolymer of polyacrylonitrile. Example 5 In a similar manner to Example 4, a portion 10 from the same knit sock was heat treated at a temperature of 1550°C. The cloth itself and the deknitted tow had a very high electrical conductivity. On carding 15 cm lengths of the cut tow, a wool-like _,,- fluff was obtained which had fiber lengths of from 2.5 to 7.5 cm with average lengths of 5 cm. Thus, carding . of a deknitted, continuous filament tow, fabric which has been subjected to a " temperature of above 1000°C is still capable of producing a wool-like fluff product. 20 Example 6 The material of Example 3 which had been heat treated to 550°C until thermoset and which possessed no c electrical conductivity was fabricated into a thermally Insulated jacket employing about 140 gm of the fluff as the sole fill of the jacket. The jacket had an insulating effect similar to that of a down jacket having from 420 to 700 gm of down as the insulating 30 fill. From 1/3 to 1/5 of the wool-like fluff of the invention therefore provides the same thermal insulation as compared to down (fowl). If desired, the fibers may be blended with other synthetic fibers such as nylon, rayon or polyester. 35 Example 7 A 3K OPF tow was knitted into a sock, the sock treated at a temperature of 525°C until it was thermally set and thereafter deknitted and cut into nominal lengths of from 18 to 19 cm. The cut tows were opened on a Shirley opener and then further processed on a Rando Webber machine (an air laying system for producing nonwoven batting). The feed plate combing " rolls were spaced apart at a distance of 0.3 mm and dispersed into the chamber using a 1200 rpm setting on the fan. A small amount of low melting fibers of ethylene acrylic acid copolymer (manufactured from PRIMAC0R 440 resin produced by The Dow Chemical Company), was blended with the cut OPF tow fibers as it was fed into the Shirley opener. The resulting batting was passed through a hot air oven and held at a temperature of 260°C at a rate of 2 m/min -resulting in an oven time of about 1 minute. This was sufficient to melt the ethylene acrylic acid copolymer to achieve a light bonding of the carbonaceous fibers in the batting. Example 8 In a similar manner described in Example 7, the cut fibers were treated in a Shirley opener and then a Rando Webber air laying system, but without the low melting polyethylene acrylic acid copolymer added. The resulting batting was processed on a Hunter Fiber Locker to obtain a mechanical bonding by the needle punching process. The resulting structure was suitable as a sound absorbing mat for use under a synthetic fiber carpet. Example 9 To establish the heat conductivity of the carbon fibers per se, two samples of a " wool-like fluff were prepared in the manner of Example 6. Each sample had a size of 20 x 20 cm 2 and a thickness of 7.5 cm. Sample 1, weighing about 43 grams, and Sample 2, weighing about 52 grams, were compressed to a thickness of 2.9 and 2.1 cm, respectively. The R-value and the K-value were measured using ASTM-C-518 method with.a 38°C hot plate and a 10°C cold plate. The results are set forth in Table V, following. TABLE V Compressed R-Value K-Value Thickness Hr-ft 2 BTU-In/Hr-ft 2 Sample *> (cm) °F/BTU °F 1 2.9 4.11 0.28 2 2.1 4.03 0.21 Sample 1 had been heat treated to a temperature of 950°C, and Sample 2 had been heated to a temperature of 550°C. Example 10 In a similar process as described in Example 9 - a 6K OPF tow was knitted, heat treated to a temperature of 550°C and deknitted. The tow was cut into 15 to 25 cm lengths, which were passed through a full production size Shirley opener and collected. A portion of this run was used in insulating an aircraft. Example 1 1 In another experiment an electrical furnace was insulated on the top section above the heater box with a wool-like fluff prepared as in Example 10. A blanket was prepared having a thickness of 20 cm and an area of 139 x 133 cm, i.e., the area above the heater box of the furnace. The fluff had a bulk density of 2 \ ~ ~/m~ . The insulating properties of this fluff was measured across a 15 cm.thick blanket by placing two thermocouples into the fluff, one thermocouple at a distance of 2.5 cm above the furnace heater and the other thermocouple at a distance of 2.5 cm below the upper extent of the fluff. The thermocouples were placed to insure that surface effects were eliminated. The temperature profile of the two thermocouples, as well as the difference between the two thermocouples, is shown in Figure 1 wherein it is illustrated that the blanket provided a temperature drop of about 350°C from the wall of the furnace to the exterior cover of the furnace. Previously the furnace required about 20.3 cm of carbon black insulation, weighing about 480 kg/m^' to obtain the same temperature drop. Accordingly, the same insulating properties were obtained using less that 0.5 percent of the bulk density of the wool-like fluff of the invention. Example 12 The noise level of a single engine p . lane, Mooney Model 20C (Manufactured by Mooney Aviation, Kerville, Texas), was measured using a sound source abutting the outside skin panel which forms the outside wall of the luggage compartment of the plane. A sound measuring meter was placed inside the plane at a dlstance of 15 cm from the inside skin of the plane. Measurements taken using several frequencies are set forth in Table VI, following. TABLE VI Frequency Inside Inside Inside Hz Decibels* Decibels** Decibels*** 250 77 59 53 500 79 63 49 1000 72 69 57 2000 86 69 51 - *No insulation **Standard lead/vinyl/fiberglass ***Present invention The plane had an original insulation package consisting of 16 kg/m^ of standard fiberglass having a thickness of 2.5 cm backed by aluminum foil positioned behind the panels of the interior of the plane. The package weighed approximately 10 kg and had a thermal resistance or R value of about 3 1/2 to 4. A package of the invention is described in Table VII, following. The total insulating area was approximately 7.5 2 . The size of the top and luggage area consisted of 5.3 m 2 which was insulated with about 5 kg of the package of the invention. The package was made up by cutting some of the batting of the invention, containing 3.2 to 3.8 cm, 500°C fibers of Example 3 with 23 percent polyester binder fiber which had been manufactured into a nonwoven batting with the use of a Rando Webber. Sections of the batting were laminated by gluing to sheets of a heavy grade aluminum foil. Each section was bagged in a Mylar reinforced film bag with the side facing the interior of the plane containing some fiberglass screening to allow for breathing of the insulation. The floor area of the plane was 1.2 m 2 . This was insulated with 850 gm of the bagged aluminum fiber structure. The material used for the floor area was a densified latex bonded carbonaceous fiber batting which was laminated to aluminum foil and placed in a similar Mylar bag. The total weight of the insulation was 5.5 kg. Of this, there was approximately 2 kg of the actual wool-like fluff of the invention, the rest of the weight was made of the aluminum foil and Mylar packing. The thermal resistivity or R value of the polyester bonded wool-like fluff of the invention was about 7-3 which is about double the value of the original insulation material used. A typical, current state-of-the-art insulation package consisting of a sound board, fiberglass and leaded vinyl would weigh 25 kg for the interior area and an additional 2 kg for the floor area, for a total package weight of 27 kg. The weight savings of the wool-like fluff batting (the package of the invention that was used) versus the standard package, which would have similar R values of from 6 to 7 . shows that the carbonaceous fluff weighed only 22 percent as much as the original package. Sound measurements were taken on this aircraft at a cruise altitude of 1500 m at standard engine -settings with the original insulation and after the new insulation package was installed. The results are shown in Table VII, following. In the speech interference level of FROM 500 to 2000 Hz, the sound value of the aircraft with the original insulation was 93-3 dB. After insulation with the new package, the -26- speech interference level value dropped to 83 dB (for every 3 dB drop the sound level is cut in one half) . Accordingly,the insulation of the Invention reduced the sound level by more than 8 fold at the ear level of the pilot. These measurements were made with the old interior of the plane fitted loosely back in for'the purpose of test flight. A new fitted interior was placed in the aircraft and the sound measurements once again measured at the 1500 level. The speech interference level at the ear level of the pilot dropped to 76.3 dB and at and elevation of 2850 m dropped even further to a value of 78.9 dB. TABLE VII - Orάgiπal Insulation Standard Insulation Structure of Insulation \ FG* Invention Structure Weight (kg) TO 5.5 27 Thermal (R) 4 8 8 Sound (SILA) 93.5 83 " 86-87 (dB) - SILA = Speech Interference Level 500 + 1K + 2K/3 *Fiberglass The study demonstrates that the sound attenuation and dampening characteristics with the carbonaceous fiber-aluminum foil-carbonaceous fiber laminated package of the invention showed a substantial improvement over conventional fiberglass/lead vinyl package of the prior art where the lead vinyl is used , to dampen sound especially at lower frequencies of less than 1000Hz. Example 1 3 Similar to Example 12, a Falcon 5 ' 0 S/N 51 airplane having an original insulation of microwhite i- fiberglass (FG) having a thickness of about 5 cm and a density of 9.6 kg/m^ was replaced with a batting of the invention. The results are shown in Table VIII, following. TABLE VIII 10 Orig. FG* Invention Std . FG- 1) Weight (kg) 67 58 110 Batting Thickness (cm) 5 10 10 15 Thermal (R) 7 14 14 Sound (SILA) 60.5 57 61 (dB) SILA = Speech Interference Le^vel 1K + 2K + 4K/3 20 *Microwhite Fiberglass Example 14 A. Carbonaceous Filaments From a Rayon Precursor- 25 A 300 denier and a 1650 denier rayon continuous tow yarn was knitted into approximately two socks having a diameter of about 2.5 cm on a single end jersey--style circular knitting machine, and were cut into four short sections. Three such sections from the 30 sock knit from the 300 denier yarn tow were introduced, one at a time, into a tube furnace. In each instance the furnace was closed and purged with nitrogen for fifteen (15) minutes. Thereafter the furnace ^ c - temperature was slowly raised for the first sock section to 370°C over a one and one half (1 1/2) hour period, for the second sock section to 550°C over a one and three quarter (1 3/4) hour period, and for the third sock section to 1050°C over a one and one quarter (1 1/4) hour period. Each section taken from the furnace was black in color. The first section which had been heated to a temperature of 370°C, was very flexible and substantially electrically nonconductive. The yarn tow was capable of careful hand deknitting. The deknitted tow was of a sinusoidal configuration and was capable of elongation to a straight (linear) length with little breakage of the individual fibers. The tow lost its sinusoidal configuration when heat was applied by blowing hot air from a heat gun (a hair dryer) against the fiber tow, thus indicating that the "set" (sinusoidal or coil-like configuration of the tow) was only temporary. Only minimal weight loss was observed as a result of the heat treatment procedure. The second section which had been heated to a temperature of 550°C was moderately flexible, was substantially electrically conductive and had an electrical resistivity of 7 x 10 0, ohms per square. The tow was capable of careful hand deknitting but broke into short lengths of from 2.5 to 5 cm. The pieces of deknitted tow had a sinusoidal configuration but were not capable of reversible full elongation (reversible deflection) without breaking, that is the individual fibers of the deknitted tow broke into short pieces even when the most gentle attempts were made to elongate the sinusoidal configuration of the tow to anything approaching a straight (liner) configuration. While the tow lengths of from 2.5 to 5 cm did not appear to lose their sinusoidal configuration when heat was applied, the fibers broke due to the force of the air from the heat gun. The yarn strands comprised 5 of the bundle of short fibers were brittle, and it was impossible, even when the most gentle conditions of handling were used, to separate the individual fibers of lengths greater than about 1 cm. 10 The third section, which had been heated to a temperature of 1050°C, was even less flexible than the previous section. It had lost over 75 percent of its original dry weight, resulting in a marked decrease of fiber diameter, and was substantially electrically 15 conductive having' an electrical resistivity of 70 ohms per square. It was not possible to draw a tow from the knitted fabric ih its knitted state after heating, even by careful hand deknitting.. The fibers broke into 20 short lengths as the tow was drawn from the fabric. On attempting to deknit the fabric, bundles of fibers of less than 1.25 cm in length having a sinusoidal configuration were not capable of elongation since the individual fibers broke into even smaller pieces. 25 B. Carbonaceous Filaments According to the Invention The procedure of Part A was followed except that instead of rayon the sock was prepared from an O Q oxidation stabilized polyacrylonitrile based (PANOX) fiber tow (3000 count filaments). The tow was heated to a temperature of 1000°C, and had a weight loss of 46.5 percent. A 5 cm length oj- of the deknitted tow had a resistance of 48 ohms. -30- A 2.5 x 5 cm section -of the sock after heating to a temperature of 1500°C had, before deknitting, a resistance of 1.9 ohms and a stretched section of a deknitted tow 2.5 cm in length had a resistance of 2.9 ohms. C. Following the procedure- of Part B, similar 10 oxidation stabilized polyacrylonitrile based (PANOX, 6000 count filaments) tow knit fabrics were heated to temperatures of 372°C and 564°C, respectively. The portion which had been heat treated to 564°C lost 31 percent of its weight and had a resistance., with 15 respect to the cloth, of 1 x 10° ohm per square. A tow drawn from the fabric had a resistance of 400 K ohms per cm. The material which had been heat treated to 20 372°C lost about 31 percent of its original weight and had an electrical resistance of greater than about 10 x 10 12 ohm per square. The experiments show that it is evident that 25 the nature of the precursor material, the oxidation stabilized polyacrylonitrile, provides properties which the rayon precursor does not provide when subjected to the same treatment. 30 Example 15 Nonflammability Test The nonflammability of the fibers of the - > - invention has been determined following the test procedure set forth in 14 FAR 25.853(b). The test was performed as follows: A minimum of three 2.5 x 15 x 15 cm specimens c - were conditioned by maintaining the specimens in a conditioning room maintained at a temperature of 70 +5 °F and 50 ± 5 percent relative humidity for 24 hours preceding the test. 10 Each specimen was supported vertically and exposed to a Bunsen or Turill burner with a nominal I.D. tube adjusted to give a flame of about 3.8 cm in height. A calibrated thermocouple pyrometer positioned in the center of the flame measured the temperature of 15 the flame. The lower edge of the specimen was about 1.9 cm above the top edge of the burner. The flame was applied to the center line of the lower edge of the specimens for 12 seconds a d then removed. 0 Pursuant to the test, the material was self- extinguishing. The average burn length did not exceed 20 cm. The average after flame did not exceed 15 seconds and no drippings of the specimens were observed. 5 Surprisingly, the fibers of the invention all had an LOI of greater than 40. 0 5

Claims: CLAIMS 1. A thermal insulating and/or sound absorbing structure comprising a batting of resilient, shape reforming, elongatable, nonlinear, nonflammable carbonaceous fibers, said fibers having a reversible deflection ratio of greater than 1.2:1, an aspect ratio greater than 10:1 and a limited oxygen index value greater than 40. 2. The structure of Claim 1, wherein said fibers are substantially electrically nonconductive. 3. The structure of Claim 2, wherein said fibers are derived from stabilized acrylic fibers having a diameter of from 7 to 20 microns, wherein a 6K tow of said fibers has an electrical resistance of greater than 4 x 10° ohms/cm. 4. The structure of Claim 3 , wherein said fibers have a nitrogen content of greater than 18.8 percent. 5. The structure of Claims 2, 3 or 4, wherein said fibers possess no antistatic characteristics. 6. The structure of Claim 1, wherein said fibers are derived from stabilized polyacrylonitrile based polymers having a nitrogen content of from 10 to 35 percent; 7. The structure of any one of the preceding claims, wherein said batting comprises a wool-like fluff having low moisture absorbency and adapted for use as insulation in clothing articles. 8 The structure of any one of the preceding claims, wherein said batting comprises a wool-like fluff in which the carbonaceous fibers have a_ sinusoidal or coil-like shape, and wherein said batting is blended..with other synthetic or natural fibers or a synthetic resinous material binder. 9. The structure of Claim 1, wherein said fibers are electrically conductive. 10. The structure of Claim 9 - wherein said fibers are derived from stabilized acrylic fibers having a carbon content of greater than 85 percent and a nitrogen content of less than 10 percent, and wherein a 6K tow of said carbonaceous fibers has an electrical resistance of less than 4 x 10- ohms/cm. 11. The structure of Claim 10, wherein said structure has a bulk density of less than 8 kg/m^. 12. The structure of Claim 1, wherein said fibers are derived from stabilized acrylic fibers having a diameter of from 7 to 20 microns, a carbon content of less than 85 percent and a nitrogen content of from 10 to 35 percent, and wherein a 6K tow of said fibers has an electrical resistance of from 4 x 10° to 4 x 10-3 ohms/cm. 13- The structure of Claim 12, wherein said j- fibers are derived from stabilized polyacrylonitrile based fibers and- have a nitrogen content of from 20 to 25 percent. 14. The structure of any one of the preceding 10 claims, wherein said fibers contain a curable polymeric binder. 15. The structure of any one of the preceding claims, wherein said structure has a bulk density of 15 from 2.4 to 32 kg/ux- . 16. The structure of any one of the preceding claims, wherein said carbonaceous fibers are made from ^ a carbonaceous precursor material selected from P0 acrylonitrile homopolymers, acrylonitrile copolymers and acrylonitrile terpolymers, wherein the acrylonitrile polymers contain at least about 85 mole percent of acrylonitrile units a ' nd up to 15 mole percent of one or more monovinyl units. 25 30 35

Inventors: HALL DAVID M, HOTCHKISS ROBERT L, MCCULLOUGH FRANCIS P JR, MCCULLOUGH JACQUELINE R, SNELGROVE R VERNON, HALL, DAVID, M., HOTCHKISS, ROBERT, L., MCCULLOUGH, FRANCIS, P., JR., MCCULLOUGH, JACQUELINE, R., SNELGROVE, R., VERNON
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1987
Main classification: B32B9
IPC codes: B32B5, B32B15, C01B31, D01F9, D04H1, G10K11