The present invention relates to a process for preparing a novel continuous, generally planar, sheet like, carbonaceous material comprised of a multiplicity of interconnected, carbonaceous fibers having a multiplicity of contact points, said fibers being integrally welded at their contact points to form said continuous, generally planar, sheet like, carbonaceous material. The carbonaceous material has a moderate to high specific resistivity, an oxygen content of less than 2 percent, a carbon content of less than 92 percent, and a nitrogen content of from 5 to 30 percent and can take a variety of physical forms, including a planar, sheet like batting, fabric, panel, web, mat, and the like.

In the process, the carbonaceous material is prepared from a meltblown or spunbonded thermoplastic polymeric precursor material. This polymeric precursor material is in the form of a multiplicity of fibers which, while in a heat softened and tacky condition, contact and adhere to each other at their contact points, thus forming a fibrous mat, web, or the like. The thus formed interconnected fibrous polymeric precursor material is then irradiated to crosslink the polymeric precursor material and heat treated in a nonoxidizing atmosphere to permanently heat set and carbonize the polymeric precursor material. The carbonaceous material of the invention provides ignition resistant, fireblocking, and thermal insulative materials.

Textile materials can be made into a variety of different physical forms, including fibers per se, a fiber assembly such as a fiber tow, mat, web, or the like. Textile materials are further exemplified in a variety of fibrous textile structures including traditional textile fibers and fabrics and microfibrous mats, webs, or the like. The Textile Institute defines a fabric as "a manufactured assembly of fibers and/or yarns, which has substantial area in relation to its thickness and sufficient mechanical strength to give the assembly inherent cohesion". Traditional textile fabrics are most commonly woven or knitted, but the term includes assemblies produced by lace-making, tufting, felting, net-making, and so called nonwoven processes. The distinctive characteristics of a fabric arises from the manner in which the fibers are arranged in a sheet-like or planar structure. Woven and knitted fabrics are made by interlacing and interlooping of assemblies of filaments or fibers (monofilament, or multifilaments with staple yarns). On the other hand, traditional nonwoven fabrics are made by blending staple fibers with a polymeric binder material followed by bonding to form a web-like array of fibers or filaments or, alternatively, mechanically

entangling the fibers such as by hydro-entanglement or lock stitching (needle punching) to form a mechanically entangled web-like array. The web-like array can be made from fibers of discreet lengths (ranging from a few millimeters to a few meters) by a carding or wet laying process. An alternative to traditional nonwoven processes for the production of microfibrous materials consists of laying or blowing filaments as they are melt extruded. The microfibrous material made by these latter processes are commonly known as spunbonded and meltblown nonwoven fabrics.

The term "fibrous material" as used herein refers to a multiplicity of randomly entangled self bonded polymeric precursor fibers or microfibers in the form or shape of a nonwoven generally planar panel, sheet, mat, web, batting, or the like. It will be apparent to persons ckilled in the art that the final shape of the randomly entangled self bonded polymeric precursor fibers or microfibers can be made to any desired specification, be it pillow shaped, rod like or the like. The fibrous material is produced using either the meltblown or spunbond process or some modification or combination thereof.

The exact final structural form of the microfibrous material can also be tailored by controlling the process depending upon the desired loft and density of the material required, i.e. a relatively thick, low density mat or a relatively thin, higher density mat. The fibrous material can also be in the form of a single ply mat or web, or a multiplicity of superimposed or stacked plies in the form of a high loft mat or batt-like structure. In other words, a spunbonded fabric can be defined generically as a continuous interconnected polymeric microfibrous fabric or material. A melt blown fabric is defined by a process in which extremely fine or "super fine" microdenier fibers of typically less than 10 microns in diameter are extruded under the influence of a dynamic flow of air and are then collected in the form of a microfibrous material on a screen or belt. As a result of the dynamic air flow, the fibers are drawn while they are still hot and tacky, so that there is obtained a difference in birefringence, crystallinity and molecular orientation as compared to conventionally spun fibers. The microdenier fibers produced in a meltblown process are much finer than those that can be produced by the traditional textile techniques of melt spinning or other traditional methods of spinning textile fibers. The meltblown microdenier fibers are bonded at the fiber to fiber intersections and contact points while the fibers are still tacky. Typical melt blown fibrous materials are usually thin, but it is possible by placing multiple extrusion heads in a close series relationship to build up an interconnected (bonded) mat or batt, for example, having a thickness of from 0.1 to 6 inches ( 2.5 to 150 mm), which is similar

in thickness to the thick batts produced from textile fibers using a traditional air-laid process. The birefrigence of meltblown and spunbonded fibers is relatively low. Meltblown fibers exhibit a lower level of birefringence than the spunbonded fibers which in turn exhibit a lower level of birefringence than traditional spun and drawn textile fibers.

The extremely fine diameter of meltblown microfibers yields a microfibrous material with a surface area of from 4 to 20 times the surface area of similar weight fibrous material containing traditional textile fibers. This phenomenon greatly increases the thermal insulation property of a meltblown polymeric microfibrous material, but, at the same time, also greatly increases the flammability of the material. For this reason, a meltblown microfibrous material is usually covered or coated on the fiber surfaces by another ignition resistant material to reduce or prevent ignition.

Processes for preparing microfibrous materials from thermoplastic materials using a meltblown process have been described in publications such as Industrial and Engineering Chemistry, Vol. 48, No. 8 (1956), pages 1342-1346. Meltblown processes are also described in U.S. Pat. Nos. 2,374,540 and 3,532,800. Methods for preparing spunbonded articles are described in U.S. Pat. Nos. 3,379,811 and 3,502,763.

U.S. Pat. No. 4,118,531, which is incorporated herein by references, discloses meltblown webs that comprise a mixture of meltblown microfibers and crimped bulking fibers wherein the mixed fibers are used for thermal insulation. These webs are sold under the tradename Thinsulate™ by Minnesota Mining and Manufacturing Corporation and are generally used as insulation for clothing articles. These webs are not irradiated nor are they heat treated to render them carbonaceous and are therefore highly flammable. As is well known, meltblown materials have found utility in a broad range of applications. For example, it is known to use polymeric meltblown filaments in the preparation of battery separators, cable wrap, capacitor insulation paper, as wrapping materials, clothing liners, diaper liners, in the manufacture of bandages and sanitary napkins, and the like. U.S. Pat. Nos. 4,837,076, 4,879, 168 by McCullough et al, which are herein incorporated by reference, disclose crimped, irreversibly heat set, carbonaceous fibers which are derived from oxidatively stabilized polyacrylonitrile fibers. These patents disclose heat treatment conditions suitable to permanently heat set the fibers in an inert atmosphere to make the fibers carbonaceous. These heat treatment conditions can be used in the process of the present invention. These fibers, however, are traditionally spun textile fibers made from acrylic (PAN) which have been oxidatively stabilized and

which contain from 5 to 20 percent by weight oxygen.

Exposing polymeric materials to ionizing radiation to alter their properties is known. Methods of radiation include X-rays, gamma-rays and electron beam (or E- Beam) radiation. These kinds of radiation are all essentially equivalent. Under exposure to radiation, free radicals or other reactive species are generated in the polymeric material. Ionizing radiation, e.g. from an electron beam generator is known to create many complex and sometimes competing reactions. For example, electron beam radiation is known to induce crosslinking of acrylonitrile.

The electron beam radiation treatment is most easily carried out at ambient temperature. There is no obstacle, however, to the radiation treatment of the fibrous material at an elevated temperature, provided that the temperature is maintained below the temperature at which the polymeric material degrades or deteriorates.

The term "carbonaceous material" used herein is understood to mean that the carbon content of the material is greater than 65 percent and less than 92 percent, the oxygen content is greater than 0 and less than 2 percent, preferably less than 1 percent, the nitrogen content is from 5 to 30 percent, and the specific resistivity of the carbonaceous material is greater than 10 ^'1 ohm-cm., more specifically from 10 ^"1 up to 10 ¹⁰ ohm-cm,

The term "carbonization" used herein is understood to mean that the carbon content of the irradiated acrylic polymeric precursor material has been increased as a result of an irreversible chemical reaction generally caused by heat treating the material in a non-oxidizing atmosphere to permanently heat set the material.

All percentages given herein are in percent by weight unless otherwise specified. According to the present invention there is provided a novel method for preparing a carbonaceous material from a meltblown or spunbonded acrylic precursor material in the form of a multiplicity of fibers or microfibers that are self bonded to each other at their contact points. The precursor material is exposed to ionizing radiation to crosslink said material and is then heat treating in an inert atmosphere for a time and temperature sufficient to increase the carbon content of said irradiated precursor material to form said carbonaceous material. The carbonaceous material thus produced consists of a multiplicity of carbonaceous fibers that are integrally welded to each other at their contact points to form a continuous fibrous carbonaceous material generally in the form or a planar sheet, fabric, panel, web, mat, or the like. The carbonaceous material is further characterized by having a carbon content of from 65 percent to less than 92 percent, a nitrogen content of from 5 percent to 30 percent, an

oxygen content of from greater than 0 percent to less than 2 percent, preferably less than 1 percent, and a specific resistivity of from 10 ^"1 to 10 ¹⁰ ohm-cm.

The spunbonded and meltblown polymeric fibers and microfibers are subjected to ionizing radiation such as by electron beam irradiation, at a dose level of less than 10 5 megarads, preferably less than 2 megarads, more preferably from 0.1 to 1.5 megarads. The effect of irradiation is to crosslink the fibrous polymeric precursor material and to stabilize the polymeric material for subsequent heat treatment in a non-oxidizing atmosphere and at a temperature and for a length of time sufficient to irreversibly convert the crosslinked precursor material to a carbonaceous material. It will be

10 readily understood by persons skilled in the art that the temperature and length of time of the heat treatment can be varied to a substantial degree and is interdependent. For example, if the temperature is increased, the time of treatment can be correspondingly reduced. If the temperature is decreased, the length of treatment can be increased to obtain the desired degree of carbonization of the crosslinked precursor material.

15 Specific haet treatment conditions useful to prepare the carbonaceous materials of the invention are described in US Patent Nos. 4,837,076 and 4,879,168, referenced herein above.

The carbonaceous material of the invention, when in the form of a batt, web or mat, generally has a bulk density of from 100 to 300 cc/g (cubic cm per gram) or,

20 conversely, 0.01 to 0.003 g/cc (grams per cubic cm), preferably from 200 to 300 cc/g (.005 to .003 g/cc). The carbonaceous batt, web or mat are particularly useful to provide high thermal insulation, with an insulation value of typically greater than 4 R/in, preferably from 5 to 6 R/in, where R is measured in hr-ft2*°F/BTU. If expressed in SI units (where R _si =mK/W), these values correspond to R _s j/m of 1134; 1417, and

25 1701, respectively. The higher insulation values are obtained from the embodiments of the invention having high microfiber content.

The carbonaceous material of the invention, when in the form of a mat or web, or the like, can be used as ignition resistant thermal insulation, padding for carpeting, etc., as articles for personal use such as gloves, jackets, sleeping bags, etc. Sheets,

30 foams, or films are particularly useful as furniture upholstery covers, curtains, comforters, mattress pads, etc.

It is therefore an object of the invention to provide a novel process for preparing an ignition resistant and/or fire-blocking, nonoxo-carbonaceous fibrous material utilizing a meltblown or spunbonded thermoplastic precursor material, where the

35 precursor fibrous material is composed of a continuous matrix of self bonded polymeric fibers or microfibers. No additional means of bonding of the fibrous material, such as a

polymeric binding agent, is necessary.

It is also an object of the invention to provide a process for preparing an ignition resistant and/or fire-blocking nonoxo-carbonaceous fibrous material which can be used alone or in combination with other fibers. It is another object to provide an ignition resistant nonoxo-carbonaceous fibrous material which can be used as thermal insulation.

It is a further object of the invention to provide a fire-blocking nonoxo- carbonaceous fibrous sheet, board or panel for use in buildings, and the like.

Other objects and advantages of the invention will become more clearly understood from the drawing and the detailed description of the preferred embodiments. Brief Description of the Drawings

Figure 1 is a schematic view of a preferred apparatus of the invention wherein a polymeric precursor material is made into a nonoxo-carbonaceous material. Description of the Preferred Embodiments

In a prefered method of manufacturing, a multiplicity of polymeric precursor fibers are extruded from a conventional spunbonding or meltblowing apparatus and are deposited on a conveyor belt. Multiple extrusion heads can be spaced closely in series to build up a textile structure of the fibrous polymeric material to any desired thickness of up to 6 inches (15 cm), for example. The textile structure of the fibrous polymeric material so formed is then passed through a source of ionizing radiation, such as an electron beam, so as to stabilize the polymeric material by crosslinking. The irradiated fibrous material is then heat treated in an oven in an inert atmosphere. The heat treatment is preferably conducted by placing the fibrous precursor material on a support such as a belt so as to produce no tension on the fibrous material, to render the crosslinked fibrous material nonoxo-carbonaceous. The nonoxo-carbonaceous fibrous material is then collected on a take-up roll or in a container for further processing. While the process is preferably continuous and can be used beginning from the initial extrusion of the fibers or microfibers, it can also be readily adapted to batchwise production. That is, the process can begin by utilizing a creel of a spunbonded or meltblown fibrous polymeric precursor material, for example, polyacrylonitrile, which has been previously crosslinked by irradiation. The crosslinked fibrous precursor material can be stored and, when convenient, passed through an oven for heat treatment of the fibrous material in an inert atmosphere to obtain partial or full carbonization. By partial carbonization is meant that the carbon content of the fibrous polymeric precursor material is increased to greater than 65 percent but less than 92

percent by heat treating in an inert atmosphere to cause permanent chemical change in the precursor material.

More particularly, Figure 1 shows a typical production line for the preparation of a generally planar fibrous material 25, such as, for example, a web or mat, comprising a multiplicity of randomly oriented carbonaceous continuous microfibers which are integrally welded at their contact points. A multiplicity of polymeric microfibers 26 are extruded from a nozzle 20 of a meltblowing apparatus and are subjected to the influence of a dynamic flow of hot air from a jet 24, as described in U.S. Pat. No. 4,1 18,531, to form a fibrous precursor material in which the melt softened fibers are bonded to each other at their contact points. A multiplicity of contact points between individual fibers are formed due to the influence of the dynamic flow of hot air on the fibers as they transverse from the extrusion nozzel to an endless conveyor 21. The fibrous material 25 is conveyed by the endless belt conveyor 21 so as to be transported past a source 27 of electron beam radiation which is normally in a protective enclosure (not shown). The fibrous polymeric precursor material is then irradiated to crosslink and stabilize the polymeric material for the subsequent carbonizing heat treatment. One or more plies each composed of a multiplicity of the self bonded polymeric precursor fibers can be extruded over the initial fibrous material with additional nozzles and air jets followed by irradiation from further electron beam sources to form a multi-plied web or mat structure. The fibrous material is then moved into an oven 30 where it is heat treated to permanently heat set and render carbonaceous under the conditions described in U.S. Pat. Nos. 4,837,076 and 4,879,168. Preferably, the fibrous material 25 is subjected to a dynamic flow of inert non-oxidizing gas during heat treatment passing from nozzle 31 through the fibrous material, thereby improving the tenacity of the microfibers by removing interstitial oxygen from the fibrous material. Preferably, the inert non-oxidizing gas is nitrogen.

The fibrous material of the invention, preferably in the form of one or more plies, can be supplied in any desired thickness depending on the particular use to be made of the web or mat and can have a thickness from 4 to 150 millimeters or greater. The density of the fibrous material can also vary widely depending on the particular uses to which the fibrous material is applied. Generally, the fibrous material has a density of at least 100 cubic centimeters/gram (cc/g).

The nonoxo-carbonaceous fibrous material of the invention can be produced into an ignition resistant insulation that meets the performance requirements as described in US Federal Test Method FTM 5903. Insulation materials can also be formed from the carbonaceous material of the invention having fire blocking

characteristics that meet the performance requirements as described in 14 CFR 25 Appendix F, Pt.III of the FAA cargo liner fireblock test and the US Navy 30 minute 500,000 BTU, 2230 °F propane burner test for fire zone protection for a ship's bulkheads. In accordance with a further embodiment of the invention, the fibrous nonoxo- carbonaceous material can be thermally bonded with a thermoplastic binder and then subjected to heat and pressure to form a flexible or rigid structural fireblocking panel or board.

In accordance with a further embodiment of the present invention, a ceramic and/or metallic coating can be formed on the nonoxo-carbonaceous material. The coated nonoxo-carbonaceous material can advantageously be used under oxidizing and high temperature applications wherein an uncoated material could otherwise not be used satisfactorily.The ceramic materials which can be utilized in the present invention are described in US Patent No. 4,902,563 issued on Feb. 20, 1990 to McCullough et al which is incorporated herein by reference.

The thickness and amount of coating applied to the carbonaceous material should be sufficient such that the surface coating substantially insulates the microfibrous substrate from the oxygen-containing atmosphere, i.e., such that the coating exposed to the oxygen-containing atmosphere protects the carbonaceous material from oxidation. The thickness and amount of coating on the carbonaceous material will depend on the form in which the material is used and the desired application for the microfibrous carbonaceous material. For example, the coating thickness will depend on whether the carbonaceous material is a single ply which can have a coating thickness of 1 micron or a batting which can have a coating thickness of 10-100 microns.

In those nonoxo-carbonaceous materials of the invention that are used as fire barriers, conformal silicone compounds, such as DC 1-2577 and commercially available from the Dow Corning Corporation, can be used as coatings on the carbonaceous material to synergistically improve their fire barrier performance as described in U.S. Pat. No. 5,024,877, issued June 18, 1991 to McCullough et al, which is incorporated herein by reference. More specifically, DC 1-2577 is a siloxane class dimethylphenylmethoxysilane supplied as a liquid comprising 73 percent oligomeric solids in 24 percent toluene and also containing 3 percent excess trimethyoxysilane. This liquid is dilutable using standard dry aromatic solvents such as toluene, xylene or mesetylene or with chlorinated solvents such as 1,1,1-trichloroethane. This material cures in air or at elevated temperature by hydrolysis to an elastoplastic silicone resin.

The precursor polymeric material used to prepare the carbonaceous material of the invention is derived from an acrylic polymer. The acrylic polymer is selected from one or more of the following: acrylonitrile based homopolymers, copolymers and terpolymers. The copolymers contain at least 85 mole percent of acrylonitrile units and 5 up to 15 mole percent of one or more monovinyl units.

Examples of vinyl monomers copolymerizable with acrylonitrile include methacrylic acid esters and acrylic acid esters such as methyl methacrylate, ethyl methacrylate,propyl methacrylate, butyl methacrylate, methyl acrylate and ethyl acrylate; vinyl esters such as vinyl acetate and vinyl propionate; acrylic acid,

10 methacrylic acid, maleic acid, itaconic acid, and the salts thereof; vinylsulfonic acid and the salts thereof.

The nonoxo-carbonaceous fibrous material of the invention derived from an acrylic precursor material can be classified according to nitrogen content and electrical specific resistivity. The nonoxo-carbonaceous material has a nitrogen content from 5 to

15 30 percent, preferably from 8 to 22 percent. Because the precursor fibrous material is stabilized or crosslinked by irradiation, it has substantially no oxygen units, i.e. less than 2 percent, preferably less than 1 percent.

In a first group, the nonoxo-carbonaceous material is partially carbonized and has a carbon content of greater than 65 percent but less than 75 percent and is 0 electrically nonconductive and does not possess any or little electrostatic dissipating characteristics, i.e., the material is not able to or poorly does dissipate an electrostatic charge. When an acrylic precursor material is used, it has a nitrogen content of from 18 to 22 percent and a specific resistivity of greater than 10 ohm-cm.

In a second group, the nonoxo-carbonaceous material is classified as 5 having low electrical conductivity, i.e. it is partially electrically conductive and has a carbon content of greater than 75 percent but less than 92 percent. When the nonoxo- carbonaceous material is derived from irradiated acrylic precusor material and has a low conductivity (high resistivity), it possesses a percentage nitrogen content of from 8 to 20 percent, preferably from 10 to 18.8 percent, and an electrical specific resistivity of 0 greater than 10 ^' ' ohm-cm..

Having thus broadly described the present invention and a preferred embodiment thereof, it is believed that the same will become even more apparent by reference to the following examples. It will be appreciated, however, that the examples are presented solely for purposes of illustration and should not be construed as limiting 5 the invention.

Example 1

An ignition resistant carbonaceous web or mat of the invention was made using a modified melt-blown process and an apparatus similar to that described in U.S.Patent No. 4,118,531. The apparatus is manufactured by J&M Laboratories, Gainesville, Ga. The molten polymer (oligomerized acrylonitrile) was extruded through a die, which had very fine holes of a diameter of the order of 0.05 mm and spaced at a distance of from 8-15 holes per centimeter. As soon as the polymer emerged from the die, it was drawn away by a stream of high velocity hot air. The temperature of the air is50°C greater than the die temperature. This web was then irradiated at less than 1.5 megarads to crosslink the polymer material of the web. The crosslinked polymeric web was then heat treated in a non-oxidizing atmosphere at 750°C to permanently heat set the polymer and render the polymer carbonaceous. The resulting carbonaceous web was an essentially continuous structure in which the carbonaceous fibers were bonded to each other and carbonized to form a very stable and cohesive structure. The resulting material had a nitrogen content of 8 percent, and an oxygen content of less than 1 percent. The carbonaceous web was tested for ignition resistance by both FTM 5903 and FAR 25.853-b vertical burn tests. The results for each test were the same, there was no afterburn, and a char length of less than 1 in (2.5 cm) was formed with no droppings. The carbonaceous material of the web had a specific resistivity of 10^ ohm- cm and a k value of 0.28 BTU in/(hr ft ² °F).

Example 2

An ignition resistant carbonaceous web of the invention was made using a modified meltblown process and an apparatus similar to that of U.S. Pat. No. 4,118,531. The apparatus is manufactured by J&M Laboratories, Gainesville, Ga. Molten polyacrylonitrile was extruded through a die having a plurality of orifices, each having a diameter of 0.4 mm. The orifices were equally spaced from each other with 10 orifices per inch (4 orifices/cm). As soon as the polymer emerges from the die, it was drawn away by a stream of hot air contacting the emerging polymer at the exit end of the orifices. The temperature of the air stream at the contact point with the polymer streams emerging from the orifices was the same as the die temperature. The web was then transported to a source of electron beam radiation where it was irradiated at an intensity of 1.5 megarads to crosslink the polymeric material of the web. The web was then heat treated in an inert atmosphere under a dynamic flow of nitrogen passing through the batting under the conditions described in U.S. Pat. No. 4,879,168 at a temperature of 550°C to be rendered carbonaceous. The carbonaceous web was

electrically non-conductive and had a specific resistivity of 10^ ohm-cm and a k value of 0.26 BTU in/(hr ft2 °F). The resulting carbonaceous web was an essentially continuous structure in which the carbonaceous fibers were bonded and carbonized at the contact points to form a very stable and cohesive structure. The nitrogen content of 5 the carbonaceous material was 18.8 percent and the oxygen content was less than 1 percent.

Example 3

A web was prepared by using the same polymer to that used in Example 2. The

10 meltblowing apparatus was operated at the same conditions previously described except that the air flow rate was increased by 20 percent during meltblowing. The increase in air flow causes an increase in the draw or attenuation of the fibers, resulting in an increase in the molecular orientation of the meltblown filaments, thereby increasing the tenacity of the filaments. The webbing that was produced contained microdenier fibers

15 having an average diameter of 5 microns which were somewhat smaller in diameter than the fibers produced in Example 2. The webbing produced in this example was then irradiated at a dosage of 2 megarads and heat treated similar to the batting of Example 2. The k value of the webbing improves from 0.26 to 0.20 BTU in/(hr ft2 °F), which was indicative of the fact that the lower the k value, the higher the degree of

20 thermal insulation.

Example 4

A carbonaceous web prepared by the same procedure used in Example 2 was placed on a conveyor belt. A polyester binder resin comprising 15 percent by weight,

25 based on the total weight of the web, in the form of a powder was distributed throughout the web using equipment produced by Nordsen Equipment Co. Samples of the web with the polyester binder were compressed as follows: Sample A was run through a set of calendar rolls which were preheated to a temperature of 150°C and produce a densified film or sheet material; Sample B was sandwiched between two

30 aluminum platens and placed in a press which was preheated to a temperature of 150°C and was compressed for 2 minutes at 100 psi pressure(689 kPa), removed from the press and was allowed to cool for 10 minutes to from a rigid compressed board. Sample A and sample B were tested for fire blocking characteristics as described in 14 CFR 25 Appendix F, Pt.III FAA cargo liner fireblock test and the US Navy 30 minute

35 500,000 BTU, 2230°F propane burner test for fire zone protection of a ship's bulkhead. Both samples successfully pass both tests demonstrating their satisfactory fire-blocking

performance in the form of densified structures under relatively severe test conditions. These excellent performance results were obtained with thin light weight structures.