Abstract:
There is disclosed a material comprising an assembly of at least one spun yarn substantially comprising carbon nanotubes, a majority of which are longer than one millimeter, such as longer than one centimeter, chemically interlinked one to another, and arranged in the morphology of spiral configurations. The disclosed materials may take the form of a yarn, thread, rope, or fabric. There are also disclosed composite materials constructed from the disclosed materials.

Description:
[0001]    This application claims the benefit of domestic priority to U.S. Provisional Patent Application Ser. No. 61/071,748 filed May 15, 2008, which is herein incorporated by reference in its entirety. 
     
    
       [0002]    Disclosed herein are materials comprising carbon nanotubes that are spun into yarns, threads, ropes, fabrics and the like. Methods of making such materials, as well as composites comprising such materials are also disclosed 
         [0003]    Metals and plastics have long been favorites for many technical applications because of their versatile physical and chemical properties including malleability, strength, durability, and/or corrosion resistance. However for an increasing number of applications, ultra-light materials exhibiting comparable or higher strength, durability and/or conductivity are needed. To date, the need for these materials has been primarily limited to high-tech applications, such as high performance aerospace and high-end electronics. However, they are becoming increasingly needed in other areas as well, such as ballistic mitigation applications (e.g. bulletproof vests, armor plating), and a wide range of commercial applications involving heat sinks, air conditioning units, computer casings, and vehicle bodies, to name a few. 
         [0004]    Recent advances in materials science and nanotechnology have led to the creation of a new class of carbon nanotube-based materials with strength to weight ratios never before possible. Carbon nanotubes and their unique properties have been known for some time. Examples of literature disclosing carbon nanotubes include, J. Catalysis, 37, 101 (1975); Journal of Crystal Growth 32, 35 (1976); “Formation of Filamentous Carbon”, Chemistry of Physics of Carbon, ed. Philip L. Walker, Jr. and Peter Thrower, Vol. 14, Marcel Dekker, Inc, New York and Base 1, 1978; and U.S. Pat. No. 4,663,230, issued Dec. 6, 1984. However, recent interest in carbon filamentary material was stimulated by a paper by lijima (1991) which made producing these materials possible. These early studies and the work that has developed from them has resulted in the discovery of a material with remarkable mechanical, electrical and thermal properties that can be produced on the industrial scale. 
         [0005]    All of the carbon nanotube yarns produced to date, using the techniques discussed above, were comprised of relatively short carbon nanotubes (&lt;1 mm), that did not specifically employ chemical-linking between adjacent carbon nanotubes in order to improve the strength of the yarn. The resulting prior art products are unable to take advantage of the full benefits associated with carbon nanotube. For example, while carbon nanotubes embedded in a polymer matrix do add some multifunctional properties to the composite, such as vibration dissipation, the polymer does not add any improved property to the nanotube itself. Indeed, it is typically difficult, if not impossible, to take advantage of the properties of the carbon nanotube, such as tensile strength, when they are dispersed in a polymer. 
         [0006]    Furthermore, the prior art does not teach covalent bonding of a substantially pure spun carbon nanotube thread with carbon nanotubes in the millimeter length range. Present carbon nanotube-based yarns, therefore, do not take advantage of the full benefits associated with carbon nanotube. Thus, there is a need to produce high strength yarns comprising carbon nanotubes that do not suffer from the deficiencies of currently available yarns, including a required polymer matrix to hold them together. 
       SUMMARY 
       [0007]    In view of the foregoing, there is disclosed a material comprising an assembly of at least one spun yarn substantially comprising carbon nanotubes, wherein a majority of the carbon nanotubes are longer than one millimeter, and are chemically interlinked one to another. In one embodiment, the carbon nanotubes are arranged in the morphology of spiral configurations. 
         [0008]    There is also disclosed materials, such as thread, rope, fabric and composite materials constructed from the carbon nanotube yarns. The unique ability to spin carbon nanotubes in the form of yarns without employing a polymer matrix between adjacent carbon nanotubes leads the inventive materials to have a wide range of application heretofore were unavailable. Such applications are able to take advantage of the novel physical and chemical properties derived from those of the carbon nanotubes. 
         [0009]    Other aspects, advantages, and novel features of the present invention will become apparent from the detailed description and drawings provided below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1 . SEM image of the raw carbon nanotube material as-received from Nanotech Labs. 
           [0011]      FIG. 2 . Schematic drawings showing generic methods for producing carbon nanotube yarn directly from aligned carbon nanotube forest. (a) The carbon nanotube forest is being spun while the yarn is drawn. (b) The aligned carbon nanotube forest is kept stationary while the yarn is being drawn and twisted. 
           [0012]      FIG. 3 . SEM images of (a) a single ply (left), a double-ply (middle), quandruple-ply (right) (b) a collection of single ply carbon nanotube thread containing chemically linked carbon nanotubes. 
           [0013]      FIG. 4 . A schematic drawing of the production of an aligned carbon nanotube thin film by rolling. Left: A piece of carbon nanotube forest impregnated with polyethylene glycol (PEG) is sandwiched between two layers of paper; middle: Rolling is used to press the carbon nanotube forest into a thin carbon nanotube film; Right: The resulting carbon nanotube thin film is sandwiched between two layers of paper. The paper was made from a mixture of glass fibers and bi-component polymer fibers. 
           [0014]      FIG. 5 . SEM images of a carbon nanotube thin film. Left: low magnification (50×). Right: high magnification (3000×). 
           [0015]      FIG. 6 . A schematic showing carbon nanotube threads being produced from aligned carbon nanotube ribbons. 
           [0016]      FIG. 7 . SEM images of two spools of carbon nanotube threads made from aligned carbon nanotube ribbons. Left: single ply thread, Right: a double ply thread. 
           [0017]      FIG. 8 . Two SEM images of a braided carbon nanotube material. 
           [0018]      FIG. 9 . A schematic drawing of a piece of carbon nanotube fabric. 
           [0019]      FIG. 10 . SEM images of carbon nanotube-based fabric made from one ply threads (left) and two ply threads (right). 
           [0020]      FIG. 11 . Chemical reactions involved in the carbon nanotube cross-linking through functionalization with vinyl-triethoxysilane. 
           [0021]      FIG. 12 . Chemical reactions involved in the carbon nanotube cross-linking through functionalization with vinyl-triethoxyaminosilane. 
           [0022]      FIG. 13 . Chemical reactions involved in the carbon nanotube functionalization with carboxyl groups followed by cross-linking with a diamine. 
           [0023]      FIG. 14 . Chemical reactions involved in the carbon nanotube carboxylation followed by thermal cross-linking. 
           [0024]      FIG. 15 . Stress-strain curves for CNT strips showing the relative mechanical behavior of the three types of media. 
       
    
    
     DETAILED DESCRIPTION 
     Definitions 
       [0025]    The term “carbon nanotubes” or “CNTs” are defined herein as crystalline structures comprised of one or many closed concentric, locally cylindrical, graphene layers. Their structure and many of their properties are described in detail in  Carbon Nanotubes: Synthesis, Structure, Properties, and Applications, Topics in Applied Physics . (Vol. 80. 2000, Springer-Verlag, M. S. Dresselhaus, G. Dresselhaus, and P. Avouris, eds.) which is herein incorporated by reference. Carbon nanotubes have demonstrated very high mechanical strengths and stiffness (Collins and Avouris, 2000, “Nanotubes for Electronics”.  Scientific American:  67, 68, and 69.) They also have very high electrical conductivity which allow current densities of more than 1,000 times that in metals (such as silver and copper). These properties, including the high specific strength and stiffness, will be beneficial to the materials disclosed herein. 
         [0026]    The term “yarn” is defined as a bundle of filaments approximately spirally arranged to form a very-high aspect ratio, approximately cylindrical structure. The filaments within the yarn are substantially parallel, in a local sense, to neighboring filaments. 
         [0027]    The phrase “carbon nanotube yarn” is a yarn composed of a plurality of carbon nanotubes. 
         [0028]    The terms “thread” and “rope” are defined as high aspect ratio, approximately cylindrical structures composed of more than one strand of yarn. The term “rope” is defined as a high aspect ratio approximately cylindrical structure composed of one yarn or thread surrounded by additional carbon nanotubes forming the mantle or outer sheath. 
         [0029]    The present disclosure relates to high-strength, materials comprising thread-like structures made from long carbon nanotubes (CNTS) and the derived materials constructed from them. More specifically, this invention relates to yarn, thread, rope, fabric and composite materials employing long CNTs, bound and twisted. 
         [0030]    Unlike the prior art, the materials of this disclosure relates to carbon nanotube yarn containing (1) long or ultra-long carbon nanotubes (&gt;1 mm), that are (2) twisted about the longitudinal axis of the yarn, and (3) chemically-linked together. The benefit of combining these three characteristic is that they allow the construction of ultra-light carbon nanotube based yarns with significantly enhanced mechanical and/or electrical properties over composite structures. 
         [0031]    The present disclosure also describe carbon nanotube based yarns, threads and ropes made from commercially available carbon nanotubes with lengths in excess of 1 mm ( FIG. 1 ), which are spirally arranged about the longitudinal axis of the yarn and chemically linked to adjacent neighboring carbon nanotubes. 
         [0032]    In one embodiment, high quality, ultra-long, such as greater than 1 mm, such as greater than 3 mm, or even greater than 1 cm, such as greater than cm and small (&lt;50 nm) diameter carbon nanotubes are used, and the degree of helicity and chemical-linking (described below) between the carbon nanotubes within the yarn is optimized to achieve high performance. In addition, fabric materials made by combining multiple strands of the disclosed yarn, thread or rope are also considered part of this disclosure. 
         [0033]    Covalent, ionic and metallic bond could be created between adjacent carbon nanotubes to achieve chemical linkage and hence to enhance the strength of yarn, thread, rope and fabric. As an example, two carbon atoms from backbone of adjacent carbon nanotubes can be bound together to create a covalent bond. Two neighbor adjacent carbon nanotubes could also be chemically linked by introducing moieties in between carbon nanotubes. 
         [0034]    Molecules and their derivatives or substances containing hydrogen, boron, carbon, nitrogen, oxygen, aluminum, silicon, phosphorus, sulfur, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, lead, and bismuth could be employed for chemically linked the adjacent carbon nanotubes via covalent, ionic and metallic bonds. 
         [0035]    In one embodiment, there may be attached to the carbon nanotubes disclosed herein, at least one functional chemical group, thereby forming a functionalized carbon nanotube. Non-limiting examples of how such functionalized carbon nanotubes may be formed are provided in U.S. Pat. Nos. 7,419,601 and 7,211,320, both of which are herein incorporated by reference. 
         [0036]    There are two methods for making yarns directly from aligned carbon nanotube forest, and their schematic drawings showing these generic methods are shown in  FIG. 2 . In the first method, the carbon nanotube forest is spun while the yarn is drawn and the in the second method the carbon nanotube forest is kept stationary while the yarn is twisted while it is being drawn. Typically carbon nanotubes with the morphology shown in  FIG. 1  could be made into yarns by this method. 
         [0037]    The first method was used for producing carbon nanotube yarn according to this disclosure. Once the preliminary yarn (called singly ply thread) is made, the yarn can be spun into multiple-ply thread. SEM images of single, double and quadruple-ply threads made in Example 1 are shown in  FIG. 3 . As described in Example 1, these threads were made from high quality carbon nanotubes and the individual carbon nanotube measures 3 to 5 mm in length. The yarn and thread shown in this disclosure incorporating the three new features: long carbon nanotubes, twisted and chemically linked together. 
         [0038]    In Example 2, an alternative way of making carbon nanotube yarns and threads are described. This is a two-step process, which comprises: (1) making thin film of aligned carbon nanotubes and (2) making yarn and thread out of the thin film. 
         [0039]    A method of making thin film with aligned carbon nanotube by rolling is shown in  FIG. 4 . In this embodiment, a piece of carbon nanotube forest impregnated with polyethyleneglycol (PEG) is initially sandwiched between two layers of paper; then a roller is used to press the carbon nanotube forest into a thin carbon nanotube film. Next, a carbon nanotube thin film will be formed sandwiched between the two layers of paper. The paper used in this process is a nonwoven paper made from a mixture of glass fibers and bi-component polymer fibers. 
         [0040]    The SEM images of the thin film made by the rolling technique are shown in  FIG. 5 . Low magnification SEM image of carbon nanotube ribbon shows that the total width of the ribbon of ˜1.5 mm. High magnification SEM image shows carbon nanotubes alignment within the film. Once the thin film is made, it can be made into yarns and threads. A schematic drawing showing carbon nanotube yarns being produced from aligned carbon nanotube ribbons is shown in  FIG. 6 . Both a single ply and a double ply carbon nanotube threads were made from the thin film shown in  FIG. 6 . 
         [0041]    In one embodiment, the yarn can be made by twisting and pulling the aligned carbon nanotube ribbons. SEM images of two spools of carbon nanotube yarn and thread made from aligned carbon nanotube ribbons is shown in  FIG. 7 . 
         [0042]    A new type of yarn made from the blended material of above disclosed carbon nanotube yarns and natural or synthetic fibers may exhibit significantly different physical properties compared to those of the original component yarns. This new blended yarn could also be used in the making of other disclosed materials in this invention for different applications. 
         [0043]    Thread and rope may be made from the yarns disclosed above primarily using two different techniques known as (1) counter-spinning and (2) braiding. Chemical-linking of carbon nanotubes between adjacent yarns may also be done after the threads and ropes have been made in order to achieve a higher degree of interaction between adjacent neighboring carbon-nanotubes strands. 
         [0044]    For the threads made by counter-spinning, some of the images have been shown in  FIG. 3  and  FIG. 7 . In addition, a braided material (braided by three strands of double spun carbon nanotube threads) was made by hand and SEM images of it are shown in  FIG. 8 . 
         [0045]    High strength fabrics may also be constructed from the above mentioned yarns, threads or ropes. These fabrics can be either woven or nonwoven. Chemical-linking of carbon nanotubes may also be done after fabrics are made in order to achieve a higher degree of mechanical attachment between adjacent neighboring yarns, threads or ropes to enhance the strength of the material. 
         [0046]    Schematic drawings and SEM images showing the structure of the carbon nanotube fabric made by weaving yarn and thread together are shown in  FIG. 9 , and  FIG. 10 , respectively. The woven fabric in  FIG. 10  was made by hand with a loom and a mixture of single ply and double ply thread was used. The diameter of the threads in the fabric is in the range of 20 to 50 um. 
         [0047]    Composite materials can also be made from the above mentioned yarns, threads, ropes and fabrics with the incorporation of other constituent materials. The constituent materials could be chosen from metals, natural and synthetic polymeric materials, ceramic materials and their combinations. There are many methods of making composites from these materials including: (1) impregnation of the carbon nanotubes with organic and/or inorganic molecules in solution; (2) dipping the materials into solutions or suspensions of organic and inorganic molecules followed by the evaporation of the solvent; (3) coating or filling the carbon nanotubes with metals, organic or other inorganic compounds in a gas phase technique. 
         [0048]    Chemical-linking between carbon nanotubes and other constituent materials could also be achieved via covalent bond, ionic bond and metallic bond. These could further improve performance by increasing the interaction between various components. 
         [0049]    The non-limiting examples of polymeric materials are chosen from single or multiple component polymers including nylon, polyurethane, acrylic, methacrylic, polycarbonate, epoxy, silicone rubbers, natural rubbers, synthetic rubbers, vulcanized rubbers, polystyrene, polyethylene terephthalate, polybutylene terephthalate, Nomex (poly-paraphylene terephtalamide), Kevlar poly (p-phenylene terephtalamide), PEEK (polyester ester ketene), Mylar (polyethylene terephthalate), viton (viton fluoroelastomer), polyetrafluoroethylene, polyetrafluoroethylene), halogenated polymers, such as polyvinylchloride (PVC), polyester (polyethylene terepthalate), polypropylene, polychloroprene, and multi-component polymers, and combination thereof. 
         [0050]    The non-limiting examples of metallic and ceramic materials that can be used in the composite materials described herein are chosen from boron carbide, boron nitride, boron oxide, boron phosphate, spinel, garnet, lanthanum fluoride, calcium fluoride, silicon carbide, carbon and its allotropes, silicon oxide, glass, quartz, aluminum oxide, aluminum nitride, zirconium oxide, zirconium carbide, zirconium boride, zirconium nitrite, hafnium boride, thorium oxide, yttrium oxide, magnesium oxide, phosphorus oxide, cordierite, mullite, silicon nitride, ferrite, sapphire, steatite, titanium carbide, titanium nitride, titanium boride, molybdenum, nickel, silver, zirconium, yttrium, and alloys or combination thereof. 
         [0051]    One of the carbon nanotube cross-linking approaches described herein could utilize silane chemistry. In this embodiment, the process could be described as: (1) attachment of vinyltrialkoxysilanes to carbon nanotube sidewall via a free radical reaction; (2) hydrolysis of the trialkoxysilane moiety; and (3) thermal process between 120-150° C. The hydroxysilane groups will form siloxane —Si—O—Si— bridges between the outer shells of adjacent nanotubes after this process ( FIG. 11 ). 
         [0052]    A similar process using amino groups for cross-linking carbon nanotubes is shown in  FIG. 12 . It is known that in acidic water solutions amines exist in protonated, that is positively charged form. Such property of amino groups grafted to nanotubes could assist in building up positive surface charge. The functionalization and cross-linking steps are shown in  FIG. 12 . 
         [0053]    Another approach for cross-linking of carbon nanotubes is shown in  FIG. 13 . The process could be described as: (1) Oxidization of carbon nanotubes in order to render their surface negatively charged due to the carboxyl groups. (2) Linkage between the carboxyl groups attached to the adjacent nanotubes via a diamine. Similar cross-linking could also be achieved by the reaction between a carboxyl group and an amino group resulting in the formation of amide moiety (also shown in  FIG. 13 ). 
         [0054]    Other than the techniques mentioned above, post treatment of the disclosed materials could be achieved via high temperature thermal annealing, passing high electric current through the disclosed materials, electron beam and/or ion radiation (chemical reactions involved in these process are shown in  FIG. 14 ). Further improvement of the thermal annealing method could be attempted by introducing additional source of carbon into the thread prior the annealing. 
         [0055]    Two of the above mentioned cross-linking approaches were employed in Example 5 and mechanical testing results from three types of materials are shown in  FIG. 15 . Clearly, mechanical performance of the materials could be enhanced as expected by the used chemical-linking approaches between carbon nanotubes. 
         [0056]    The above mentioned yarns, threads or ropes made with carbon nanotubes having differing characteristics can be woven together to create unique materials that take advantage of the incredibly diverse properties of the carbon nanotube. For example, depending on the application, carbon nanotubes that exhibit unique electrical, thermal, electromagnetic, strength, and filtration/detection properties can be combined in a yarn to be woven into a multifunctional material. 
         [0057]    The invention will be further clarified by the following non-limiting examples, which is intended to be purely exemplary of the invention. 
       EXAMPLES 
     Example 1 
     Carbon Nanotube Yarn and Thread from Dry Process 
       [0058]    Raw carbon nanotubes were provided by NanoTech Labs (Yadkinville, N.C. 27055) in clusters typically measuring 3 to 5 mm thickness, 1-2 cm long and 1-2 cm wide. They were used for carbon nanotube yarns making with individual carbon nanotube measuring 3-5 mm in length. Yarns according to this example were made by: a) continuously and sequentially pulling carbon nanotubes from the as-received carbon nanotube clusters; b) twisting the carbon nanotube fibers to make the yarn; c) winding the resulting yarn on to the collecting spool; d) carboxyl functionalization of the spool of yarn; e) heat treating at 500° C. for 30 min to achieve cross-linking within the yarn. The twisting and collection was performed automatically to achieve uniformity. 
         [0059]    The yarns shown in  FIG. 3  were made by using the first method (shown in  FIG. 2 ), which comprised spinning the carbon nanotube forest while the yarn was drawn. By using counter-spinning technique, the yarn (also called singly ply thread) could be spun into multiple-ply thread. SEM images of single, double and quadruple-ply threads are shown in  FIG. 3 . These threads were made from high quality carbon nanotubes and the individual carbon nanotube measures 3 to 5 mm in length. The yarn and thread shown in this disclosure incorporating the three new features: long carbon nanotubes, twisted and chemically linked together. 
       Example 2 
     Wet Spun Carbon Nanotube Yarns 
       [0060]    The carbon nanotube yarns according to this example were produced by: a) impregnating carbon nanotube material with PEG-2000; b) removing the excess PEG from the carbon nanotube material to make carbon nanotube dough; c) sandwiching the resulting carbon nanotube dough between two layers of paper; d) producing thin film by repeatedly running roller over the carbon nanotube dough; e) slitting the carbon nanotube thin film into narrow ribbons; f) twisting the narrow ribbons into yarns; g) baking the resulting yarns at 220° C. for half an hour; h) carboxylation of the spool of yarn; i) heating at 500° C. for 30 mins to achieve cross-linking within the yarn. 
         [0061]    The method for making carbon nanotube thin film is depicted in  FIG. 4  and the SEM images of the resulted thin film are shown in  FIG. 5 . Low magnification SEM image of carbon nanotube ribbon shows a total width of the ribbon of ˜1.5 mm. High magnification SEM image is showing carbon nanotubes alignment within the film. 
         [0062]    The method for making carbon nanotube yarns from aligned carbon nanotube ribbons is depicted in  FIG. 7 . SEM images of two spools of carbon nanotube yarn and thread made from the above aligned carbon nanotube ribbons are shown in  FIG. 8 . These yarn and thread were made by twisting and pulling the aligned carbon nanotube ribbons and both a single ply and a double ply carbon nanotube yarn and thread were made from the thin film shown in  FIG. 6 . 
       Example 3 
     Braided Carbon Nanotube Materials 
       [0063]    By using the techniques shown in example 1, some double ply threads were made. Using the conventional technique, under optical microscope, a piece of braided material was made by tweezers. Two SEM images of a braided carbon nanotube material are shown in  FIG. 8  and three strands of double spun carbon nanotube yarns were used in this braided material. 
       Example 4 
     Carbon Nanotube Fabric 
       [0064]    By using the techniques shown in example 1, some single ply and double ply threads were made. A schematic drawing of a piece of carbon nanotube fabric is shown in  FIG. 9 . Under optical microscope, a home made loom was used for the weaving of the fabric. SEM images of the piece of woven fabric are shown in  FIG. 10 . The fabric was woven from a mixture of single and double spun carbon nanotube yarns. The diameter of the yarns is in the range of 20 to 50 um. 
       Example 5 
     Chemical-Linking of Carbon Nanotubes 
       [0065]    The experiments on cross-linking of carbon nanotubes were performed over carbon nanotube strips. The same process could be applied to the disclosed materials in this invention. 
         [0066]    Long CNTs (3-5 mm in length) with diameters of 30-50 nm provided by NanoTechLabs were used as received. The detail procedure of the experiments is described as: 
         [0067]    I. Thermal Annealing 
         [0068]    (1) Long CNTs were acid washed and dispersed. 
         [0069]    (2) Suspension of carbon nanotubes were deposited onto carbon cloth substrate discs. 
         [0070]    (3) Carbon nanotube membrane was peeled off the substrate, pressed with a hand roller and dried. 
         [0071]    (4) Seven thin strips of roughly 0.25 mm thickness were slit from the central part of each membrane. These strips were called untreated. 
         [0072]    (5) Four of the seven strips were annealed at 500° C. for half an hour. These strips were called heat treated. 
         [0073]    II. Chemical Treatment 
         [0074]    (1) Vinyltrialkoxysilanes were attached to the long carbon nanotube sidewall via free radical reaction. 
         [0075]    (2) Functionalized carbon nanotubes from step 1 were dispersed. 
         [0076]    (3) Suspension of carbon nanotubes were deposited onto carbon cloth substrate discs. 
         [0077]    (4) Carbon nanotube membrane was peeled off the substrate, pressed with a hand roller and dried. 
         [0078]    (5) Carbon nanotube membrane was thermal processed at 120-150° C. to form siloxane —Si—O—Si— bridges between the outer shells of the adjacent nanotubes. 
         [0079]    All 10 strips were tested with an MTS Insight Tensile Tester under uniaxial tensile loading and the stress-strain curves for each strip are shown in  FIG. 15 . The early mechanical behavior of both types of cross-linked strips is very similar (nearly equal slope) with the chemically linked strips being able to withstand higher applied stresses. Both types of treated strips were shown to consistently carry a higher tensile loading before breaking and have a steeper stress-strain relationship, conclusively demonstrating an improvement in the mechanical behavior in tensile strength. 
         [0080]    Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present invention. It is intended that the specification and examples be considered as exemplary only, with the true scope of the invention being indicated by the following claims.