WO2008140567A2 - Microwave processing of ballistic composites - Google Patents

Abstract

The present invention relates to the production of ballistic resistant articles. Prior to molding, the ballistic resistant fabrics are heated with microwave energy as an alternative to conventional preheating heating methods, reducing heating time and increasing produciton efficiency.

Description

MICROWAVE PROCESSING OF BALLISTIC COMPOSITES

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates to the production of moldable, ballistic resistant articles. As part of the molding process, ballistic resistant fabrics are heated with microwave energy as an alternative to conventional preheating heating methods, reducing heating time and increasing production efficiency.

DESCRIPTION OF THE RELATED ART Ballistic resistant articles containing high strength fibers that have excellent properties against high speed projectiles are known. Articles such as bullet resistant vests, helmets, vehicle panels and structural members of military equipment are typically made from fabrics comprising high strength fibers. High strength fibers conventionally used include polyethylene fibers, para-aramid fibers such as poly(phenylenediamine terephthalamide), graphite fibers, nylon fibers, glass fibers and the like. For many applications, such as vests or parts of vests, the fibers may be used in a woven or knitted fabric. For many of the other applications, the fibers are encapsulated or embedded in a matrix material to form either rigid or flexible fabrics.

Various ballistic resistant constructions are known that are useful for the formation of articles such as helmets, panels and vests. For example, U.S. patents 4,403,012, 4,457,985, 4,613,535, 4,623,574, 4,650,710, 4,737,402, 4,748,064, 5,552,208, 5,587,230, 6,642,159, 6,841,492, 6,846,758, all of which are incorporated herein by reference, describe ballistic resistant composites which include high strength fibers made from materials such as extended chain ultrahigh molecular weight polyethylene. These composites display varying degrees of resistance to penetration by high speed impact from projectiles such as bullets, shells, shrapnel and the like.

For example, U.S. patents 4,623,574 and 4,748,064 disclose simple composite structures comprising high strength fibers embedded in an elastomeric matrix. U.S. patent 4,650,710 discloses a flexible article of manufacture comprising a plurality of flexible layers comprised of high strength, extended chain polyolefin (ECP) fibers. The fibers of the network are coated with a low modulus elastomeric material. U.S. patents 5,552,208 and 5,587,230 disclose an article and method for making an article comprising at least one network of high strength fibers and a matrix composition that includes a vinyl ester and diallyl phthalate. U.S. patent 6,642,159 discloses an impact resistant rigid composite having a plurality of fibrous layers which comprise a network of filaments disposed in a matrix, with elastomeric layers there between. The composite is bonded to a hard plate to increase protection against armor piercing projectiles.

In general, ballistic resistant articles are formed by molding a combination of fibers, any matrix composition and any additional polymeric layers in a desired configuration by subjecting the combination to heat and pressure for a particular mold cycle time. In the molding process, it is very important that the molding temperature is low enough to avoid damage to the component fibers of the ballistic resistant fabric. Particularly, is very important that the molding temperature is less than the melting point of the polymer from which the fibers are formed or the temperature at which fiber damage occurs. For example, for extended chain polyethylene fibers, such as SPECTRA® fibers manufactured by Honeywell International, Inc., molding temperatures of about 68⁰F (20⁰C) to about 293°F (145°C) are generally acceptable. However, the fibers are affected after prolonged exposure to heat above 265°F (129°C), and SPECTRA® fibers melt at 300⁰F (149°C). Temperatures higher than 265°F may cause the filaments of the fibrous layers to deform, reducing their ballistic resistance properties. Other fiber types may be able to tolerate higher molding temperatures. For example, for aramid fibers, the upper limitation of the temperature range is generally about 20⁰C to about 30⁰C higher than for extended chain polyethylene fibers.

Furthermore, in an efficient process, it is also desired to maximize the molding temperature to minimize molding times. Using common convection heating, standard molding times range from about 20 to 60 minutes at a temperature range of about 176°F (8O⁰C) to about 293°F (145°C) and at a pressure of from about 10 psi (69 kPa) to about 10,000 psi (69,000 kPa). Moreover, common convection heating requires the molding machine to be pre-heated for at least 10 minutes prior to molding. Accordingly, there is a need in the art for a more efficient fabric molding process that can effectively form ballistic resistant articles at a relatively low temperature with a relatively short molding time.

The present invention provides a solution to this need in the art. The invention presents a method of forming ballistic resistant articles with reduced heating time in preparation for molding of said articles. Ballistic resistant fabrics, such as Spectra Shield® fabrics manufactured by Honeywell International, Inc., are generally poor conductors of heat. Accordingly, a long pre-heating time is necessary before such fabrics are hot enough to be moldable into ballistic resistant articles. By reducing this pre-heating time, production efficiency can be significantly improved. The invention provides a method of molding a ballistic resistant article comprising heating a ballistic resistant fabric with microwave energy followed by molding the heated fabric. By heating with microwave energy, the overall heating and molding time is significantly reduced. The inventive process is more efficient than conventional heating techniques, and allows for considerably improved productivity. Additionally, when using conventional heat sources, long exposures to high temperatures are necessary to insure good bonding between the fibers and can result in fiber degradation. Microwave processing avoids this problem by allowing short heating times and avoiding significant temperature gradients within the samples due to even distribution of microwave energy and heating uniformity.

SUMMARY OF THE INVENTION

The invention provides a method of forming an article comprising: a) providing a fabric comprising a plurality of fibers arranged in an array, said fibers having a tenacity of about 7 g/denier or more and a tensile modulus of about 150 g/denier or more; said fibers having an optional microwave-reactive composition thereon; and b) heating said fabric inside a microwave oven by subjecting the fabric to microwave energy sufficient to thereby heat the fibers or the optional microwave- reactive composition to at least about the softening temperature of the fibers or the softening temperature of the optional microwave-reactive composition; c) molding the heated fabric into an article while said fabric has a temperature of at least about the softening temperature of the fibers or the softening temperature of the optional microwave-reactive composition due to the application of microwave energy; and d) allowing the molded fabric to cool. The invention also provides a method of forming a consolidated fiber network, said consolidated network of fibers comprising a plurality of fiber layers, each fiber layer comprising a plurality of fibers having a tenacity of about 7 g/denier or more and a tensile modulus of about 150 g/denier or more; and said fibers having a polymeric matrix composition thereon; which consolidated fiber network is consolidated under heat and pressure, wherein the heat of consolidation is generated by the application of microwave energy sufficient to thereby heat the polymeric matrix composition to a temperature of at least about the softening temperature of polymeric matrix composition.

The invention further provides a ballistic resistant article comprising a ballistic resistant fabric, the ballistic resistant fabric comprising a plurality of fibers arranged in an array, said fibers having a tenacity of about 7 g/denier or more and a tensile modulus of about 150 g/denier or more; said fibers having a dry, microwave-reactive composition coated thereon, which microwave-reactive composition has been heated above its softening point temperature by the application of microwave energy.

DETAILED DESCRIPTION OF THE INVENTION Microwave ovens provide an effective way of uniformly heating many non- conductive materials, such as ballistic resistant fabrics. Microwave processing of ballistic resistant fabrics provides desirable benefits, including economic benefits through the saving of energy and time, and increased process yield and throughput. The materials produced herein ostensibly have uniquely uniform microstructures that are not achieved by other heating methods due to the even energy distribution and uniform heating from the microwave. For the purposes of the invention, fabrics having superior ballistic penetration resistance describe those which exhibit excellent properties against high speed projectiles. As used herein, a "fiber" is an elongate body the length dimension of which is much greater than the transverse dimensions of width and thickness. The cross-sections of fibers for use in this invention may vary widely. They may be circular, flat or oblong in cross-section. Accordingly, the term fiber includes filaments, ribbons, strips and the like having regular or irregular cross-section. They may also be of irregular or regular multi-lobal cross-section having one or more regular or irregular lobes projecting from the linear or longitudinal axis of the fibers. Most commonly, fibers are single lobed and have a substantially circular cross-section.

As used herein, a "yarn" is a strand of interlocked fibers. A "parallel array" describes an orderly parallel arrangement of fibers or yarns. A fiber "layer" describes a planar arrangement of woven or non- woven fibers or yarns. A fiber "network" denotes a plurality of interconnected fiber or yarn layers. A fiber network can have various configurations. For example, the fibers or yarn may be formed as a felt or another woven, non- woven or knitted, or formed into a network by any other conventional technique. In general, a "fabric" may relate to either a woven or non-woven material, or a combination thereof. As used herein, the term "fabric" describes structures including multiple fibrous layers either before or after molding to form a composite.

As used herein, a "high-strength, high tensile modulus fiber" is one which has a preferred tenacity of at least about 7 g/denier or more, a preferred tensile modulus of at least about 150 g/denier or more, both as measured by ASTM D2256 and preferably an energy-to-break of at least about 8 J/g or more. As used herein, the term "denier" refers to the unit of linear density, equal to the mass in grams per 9000 meters of fiber or yarn. As used herein, the term "tenacity" refers to the tensile stress expressed as force (grams) per unit linear density (denier) of an unstressed specimen. The "initial modulus" of a fiber is the property of a material representative of its resistance to deformation. The term "tensile modulus" refers to the ratio of the change in tenacity, expressed in grams-force per denier (g/d) to the change in strain, expressed as a fraction of the original fiber length (in/in).

Particularly suitable high-strength, high tensile modulus fiber materials include extended chain polyolefin fibers, such as highly oriented, high molecular weight polyethylene fibers, particularly ultra-high molecular weight polyethylene fibers, and ultra-high molecular weight polypropylene fibers. Also suitable are extended chain polyvinyl alcohol fibers, extended chain polyacrylonitrile fibers, para- aramid fibers, polybenzazole fibers, such as polybenzoxazole (PBO) and polybenzothiazole (PBT) fibers and liquid crystal copolyester fibers. Each of these fiber types is conventionally known in the art.

In the case of polyethylene, preferred fibers are extended chain polyethylenes having molecular weights of at least 500,000, preferably at least one million and more preferably between two million and five million. Such extended chain polyethylene (ECPE) fibers may be grown in solution spinning processes such as described in U.S. patent 4,137,394 or 4,356,138, which are incorporated herein by reference, or may be spun from a solution to form a gel structure, such as described in U.S. patent 4,551,296 and 5,006,390, which are also incorporated herein by reference.

The most preferred polyethylene fibers for use in the invention are polyethylene fibers sold under the trademark Spectra® from Honeywell International Inc. Spectra® fibers are well known in the art and are described, for example, in commonly owned U.S. patents 4,623,547 and 4,748,064 to Harpell, et al. Ounce for ounce, Spectra® high performance fiber is ten times stronger than steel, while also light enough to float on water. The fibers also possess other key properties, including resistance to impact, moisture, abrasion chemicals and puncture.

Suitable polypropylene fibers include highly oriented extended chain polypropylene (ECPP) fibers as described in U.S. patent 4,413,110, which is incorporated herein by reference. Suitable polyvinyl alcohol (PV-OH) fibers are described, for example, in U.S. patents 4,440,711 and 4,599,267 which are incorporated herein by reference. Suitable polyacrylonitrile (PAN) fibers are disclosed, for example, in U.S. patent 4,535,027, which is incorporated herein by reference. Each of these fiber types is conventionally known and are widely commercially available.

Suitable aramid (aromatic polyamide) or para-aramid fibers are commercially available and are described, for example, in U.S. patent 3,671,542. For example, useful poly(p-phenylene terephthalamide) filaments are produced commercially by Dupont corporation under the trade name of KEVLAR®. Also useful in the practice of this invention are poly(m-phenylene isophthalamide) fibers produced commercially by Dupont under the trade name NOMEX®. Suitable polybenzazole fibers for the practice of this invention are commercially available and are disclosed for example in U.S. patents 5,286,833, 5,296,185, 5,356,584, 5,534,205 and 6,040,050, each of which are incorporated herein by reference. Preferred polybenzazole fibers are ZYLON® brand fibers from Toyobo Co. Suitable liquid crystal copolyester fibers for the practice of this invention are commercially available and are disclosed, for example, in U.S. patents 3,975,487; 4,118,372 and 4,161,470, each of which is incorporated herein by reference.

The other suitable fiber types for use in the present invention include glass fibers, fibers formed from carbon, fibers formed from basalt or other minerals, M5® fibers and combinations of all the above materials, all of which are commercially available. M5® fibers are manufactured by Magellan Systems International of Richmond, Virginia and are described, for example, in U.S. patents 5,674,969, 5,939,553, 5,945,537, and 6,040,478, each of which is incorporated herein by reference.

As stated above, a high-strength, high tensile modulus fiber is one which has a preferred tenacity of about 7 g/denier or more, a preferred tensile modulus of about 150 g/denier or more and a preferred energy-to-break of about 8 J/g or more, each as measured by ASTM D2256. For greater ballistic resistance properties, the tenacity of the fibers should be about 15 g/denier or more, preferably about 20 g/denier or more, more preferably about 25 g/denier or more and most preferably about 30 g/denier or more; the fibers preferably also have a tensile modulus of about 300 g/denier or more, more preferably about 400 g/denier or more, more preferably about 500 g/denier or more, more preferably about 1,000 g/denier or more and most preferably about 1,500 g/denier or more. Fibers with increased ballistic protection properties also have a preferred energy- to-break of about 15 J/g or more, more preferably about 25 J/g or more, more preferably about 30 J/g or more and most preferably have an energy-to-break of about 40 J/g or more. These combined high strength properties are obtainable by employing well known solution grown or gel fiber processes. U.S. patents 4,413,110, 4,440,711, 4,535,027, 4,457,985, 4,623,547 4,650,710 and 4,748,064 generally discuss the preferred high strength, extended chain polyethylene fibers employed in the present invention, and their disclosures are incorporated herein by reference.

Ballistic resistant fabrics may comprise one or more woven or non-woven fibrous layers, or a combination thereof. Woven and non-woven fibrous layers may be formed using techniques that are commonly known in the art. Suitable non- woven fibrous layers include those comprising randomly oriented fibers, as with a felt, and a plurality of fibers or yarns arranged in a substantially parallel array. In a common construction, the non- woven fibrous layers of the invention comprise a single-layer, consolidated network of fibers in an elastomeric or rigid polymer composition, referred to in the art as a matrix composition. In general, a "polymeric matrix composition" is a binder material that binds the fibers together after a consolidation or lamination step. A "consolidated network" describes a consolidated combination of multiple fiber layers with the matrix composition. As used herein, a "single layer" structure refers to structure composed of one or more individual fiber layers that have been consolidated into a single unitary structure, wherein consolidation can occur via drying, cooling, heating, pressure or a combination thereof. The consolidated network may also comprise a plurality of yarns that are coated with such a matrix composition, formed into a plurality of layers and consolidated into a single fabric layer.

In either a random or parallel non- woven parallel orientation, the individual fibers forming the fabric layer may or may not be coated on, impregnated with, embedded in, or otherwise applied with a matrix composition, using well known techniques in the art. The matrix composition may be applied to the high strength fibers either before or after the layers are formed, then followed by consolidating the matrix material-fibers combination together to form a multilayer complex. Most particularly, the non- woven fibrous layers of the invention comprise: i) a plurality of layers, each layer comprising a plurality of unidirectionally aligned, parallel fibers, wherein said layers are cross-plied at an angle relative to a longitudinal fiber direction of each adjacent fiber layer; and wherein said fibers optionally have a polymeric matrix composition thereon; or ii) one or more layers comprising a plurality of randomly aligned fibers; and wherein said fibers optionally have a polymeric matrix composition thereon.

As is conventionally known in the art, non-woven fabrics achieve excellent ballistic resistance when the individual component fiber layers are cross-plied such that the fiber alignment direction of one layer is rotated at an angle with respect to the fiber alignment direction of another layer, such that they are non- parallel. For example, a preferred structure has two fiber layers of the invention positioned together such that the longitudinal fiber direction of one layer is perpendicular to the longitudinal fiber direction of the other layer. In another example, a five layered structure is formed in which the second, third, fourth and fifth layers are rotated +45°, - 45°, 90° and 0°, with respect to the first layer, but not necessarily in that order. For the purposes of this invention, adjacent layers may be aligned at virtually any angle between about 0° and about 90° with respect to the longitudinal fiber direction of another layer, but the about 0° and about 90° fiber orientations are preferred. While the examples above illustrate fabrics that include either two or five individual fiber layers, such is not intended to be limiting. The non-woven fibrous layers can be constructed via a variety of well known methods, such as by the methods described in U.S. patent 6,642,159. It should be understood that the single-layer consolidated networks of the invention may generally include any number of cross-plied layers, such as about 20 to about 40 or more layers as may be desired for various applications. Woven fibrous layers may be formed using techniques that are well known in the art using any fabric weave, such as plain weave, crowfoot weave, basket weave, satin weave, twill weave and the like. Plain weave is most common. Prior to weaving, the individual fibers of each woven fibrous material may or may not be coated with a polymeric matrix composition in a similar fashion as the non-woven fibrous layers using the same matrix compositions as the non-woven fibrous layers.

Alternately, the fabrics may comprise a hybrid combination of alternating or non- alternating woven and non- woven fibrous layers, such as a non- woven/woven/non-woven or woven/non-woven/woven structure. Ballistic resistant fabrics may include any number of combined woven and/or non- woven layers, and each non-woven layer may comprise single-layer consolidated networks which incorporate multiple component layers. Adjacent layers may optionally be attached with an intermediate adhesive layer. Each woven layer, in particular, is preferably attached to an adjacent layer via an adhesive layer.

Suitable adhesives non-exclusively include elastomeric materials such as polyethylene, cross-linked polyethylene, chlorosulfonated polyethylene, ethylene copolymers, polypropylene, propylene copolymers, polybutadiene, polyisoprene, natural rubber, ethylene-propylene copolymers, ethylene-propylene-diene terpolymers, polysulfide polymers, polyurethane elastomers, polychloroprene, plasticized polyvinylchloride using one or more plasticizers that are well known in the art (such as dioctyl phthalate), butadiene acrylonitrile elastomers, poly (isobutylene-co-isoprene), polyacrylates, polyesters, unsaturated polyesters, polyethers, fiuoroelastomers, silicone elastomers, copolymers of ethylene, thermoplastic elastomers, phenolics, polybutyrals, epoxy polymers, styrenic block copolymers, such as styrene-isoprene-styrene or styrene-butadiene-styrene types, and other suitable adhesive compositions conventionally known in the art. Particularly preferred adhesive compositions include methacrylate adhesives, cyanoacrylate adhesives, UV cure adhesives, urethane adhesives, epoxy adhesives, ethylene vinyl acetate adhesives and blends of the above materials. Most preferably, the adhesive comprises a thermoplastic polymer, particularly ethylene vinyl acetate. Such adhesives may be applied, for example, in the form of a hot melt, film, paste or spray, or as a two-component liquid adhesive.

The woven or non- woven fibrous layers of the invention may be prepared using a variety of matrix materials, including both low modulus, elastomeric matrix materials and high modulus, rigid matrix materials. Suitable matrix materials non-exclusively include low modulus, elastomeric materials having an initial tensile modulus less than about 6,000 psi (41.3 MPa), and high modulus, rigid materials having an initial tensile modulus at least about 300,000 psi (2068 MPa), each as measured at 37°C by ASTM D638. As used herein throughout, the term tensile modulus means the modulus of elasticity as measured by ASTM 2256 for a fiber and by ASTM D638 for a matrix material.

An elastomeric matrix composition may comprise a variety of polymeric and non- polymeric materials. A preferred elastomeric matrix composition comprises a low modulus elastomeric material. For the purposes of this invention, a low modulus elastomeric material has a tensile modulus, measured at about 6,000 psi (41.4 MPa) or less according to ASTM D638 testing procedures. Preferably, the tensile modulus of the elastomer is about 4,000 psi (27.6 MPa) or less, more preferably about 2400 psi (16.5 MPa) or less, more preferably 1200 psi (8.23 MPa) or less, and most preferably is about 500 psi (3.45 MPa) or less. The glass transition temperature (Tg) of the elastomer is preferably less than about 0⁰C, more preferably the less than about -40⁰C, and most preferably less than about - 50⁰C. The elastomer also has an preferred elongation to break of at least about 50%, more preferably at least about 100% and most preferably has an elongation to break of at least about 300%.

A wide variety of matrix materials and formulations having a low modulus may be utilized as the matrix. Representative examples include polybutadiene, polyisoprene, natural rubber, ethylene-propylene copolymers, ethylene-propylene- diene terpolymers, polysulfϊde polymers, polyurethane elastomers, chlorosulfonated polyethylene, polychloroprene, plasticized polyvinylchloride, butadiene acrylonitrile elastomers, poly(isobutylene-co-isoprene), polyacrylates, polyesters, polyethers, fluoroelastomers, silicone elastomers, copolymers of ethylene, and combinations thereof, and other low modulus polymers and copolymers curable below the melting point of the polyolefin fiber. Also preferred are blends of different elastomeric materials, or blends of elastomeric materials with one or more thermoplastics.

Particularly useful are block copolymers of conjugated dienes and vinyl aromatic monomers. Butadiene and isoprene are preferred conjugated diene elastomers. Styrene, vinyl toluene and t-butyl styrene are preferred conjugated aromatic monomers. Block copolymers incorporating polyisoprene may be hydrogenated to produce thermoplastic elastomers having saturated hydrocarbon elastomer segments. The polymers may be simple tri-block copolymers of the type A-B-A, multi-block copolymers of the type (AB)_n (n= 2-10) or radial configuration copolymers of the type R-(BA)_x (x=3-150); wherein A is a block from a polyvinyl aromatic monomer and B is a block from a conjugated diene elastomer. Many of these polymers are produced commercially by Kraton Polymers of Houston, TX and described in the bulletin "Kraton Thermoplastic Rubber", SC-68-81. The most preferred matrix polymer comprises styrenic block copolymers sold under the trademark KRATON® commercially produced by Kraton Polymers. The most preferred low modulus matrix composition comprises a polystyrene- polyisoprene-polystrene-block copolymer.

Preferred high modulus, rigid matrix materials useful herein include materials such as a vinyl ester polymer or a styrene-butadiene block copolymer, and also mixtures of polymers such as vinyl ester and diallyl phthalate or phenol formaldehyde and polyvinyl butyral. A particularly preferred rigid matrix material for use in this invention is a thermosetting polymer, preferably soluble in carbon- carbon saturated solvents such as methyl ethyl ketone, and possessing a high tensile modulus when cured of at least about 1x10 psi (6895 MPa) as measured by ASTM D638. Particularly preferred rigid matrix materials are those described in U.S. patent 6,642,159, which is incorporated herein by reference.

The rigidity, impact and ballistic properties of the articles formed from the fabric composites of the invention are affected by the tensile modulus of the matrix polymer. For example, U.S. patent 4,623,574 discloses that fiber reinforced composites constructed with elastomeric matrices having tensile moduli less than about 6000 psi (41,300 kPa) have superior ballistic properties compared both to composites constructed with higher modulus polymers, and also compared to the same fiber structure without a matrix. However, low tensile modulus matrix polymers also yield lower rigidity composites. Further, in certain applications, particularly those where a composite must function in both anti-ballistic and structural modes, there is needed a superior combination of ballistic resistance and rigidity. Accordingly, the most appropriate type of matrix polymer to be used will vary depending on the type of article to be formed from the fabrics of the invention. In order to achieve a compromise in both properties, a suitable matrix composition may combine both low modulus and high modulus materials to form a single matrix composition.

Ballistic resistant fabrics may be used for various applications. For example, one or more fabrics of the invention may be arranged together to form flexible articles, including garments such as vests, pants, hats, or other articles of clothing, as is well known in the art. The fabrics of the invention may also be formed into other personal protective articles such as helmets, or may be formed into protective shields, covers or blankets as desired. Other common structures include flat, planar panels or customized shaped panels. Molded fabrics may be used, for example, to fortify armored civilian vehicles for NIJ Level I, IIA, II, IIIA and III protection; as armored doors and roofs for police cars and other vehicles; as trauma pads or breast plate inserts for ballistic resistant vests for NIJ Level I, IIA, II, IIIA and III protection; for hand-held riot shields at NIJ Level I, IIA, II, IIIA and III protection, or for explosion management devices. Multiple fabrics may be stacked or arranged in a bonded or non-bonded array. Bonding may be done using any conventional means in the art, such as stitching or bonding together with adhesive materials, other thermoplastic materials, or non- thermoplastic fibers or materials.

In order to produce such desired articles from ballistic resistant fabrics, the fabrics are subjected to a process referred to in the art as molding, hi a typical molding process, a fabric, which may comprise any number of woven and/or non- woven layers (also referred to as "plies"), is heated or pre-heated to a desired molding temperature which allows it to be formed into a shaped article or panel. The heated fabric is either shaped or compressed in a suitable molding apparatus, typically under pressure. Typical molding pressures range from about 50 psi to about 5000 psi, more commonly from about 200 psi to about 1500 psi. The molding step may take from about 4 seconds to about 45 minutes.

In accordance with the invention, the fabric is heated in a microwave oven instead of by any other traditional heating method. Microwave heating is a more efficient alternative to conventional heating because of its efficient volumetric heat production. Volumetric heating is defined as heating of the bulk, as opposed to transferring heat inward from the surface. Microwaves cause heating within a material by generating waves that excite molecules, causing them to rotate. Any molecule which is "polar" and has positive and negative ends will be rotated back and forth to align with the changing electric field of the waves in the oven. This rotation produces energy in the form of heat. Unlike conventional heating, this effect occurs simultaneously throughout the whole material being microwaved. Microwaves are electromagnetic waves in the frequency band from 300 MHz (3 x 108 cycles/second) to 300 GHz (3 ^χ 1011 cycles/second). These two frequencies correspond to wavelengths of 1 m and 1 mm, respectively. All domestic microwave ovens and laboratory microwave processors operate at 2.45 GHz, corresponding to a wavelength of about 12.2 cm. Industrial microwaves may operate at a 2.45 GHz frequency, and may also operate at lower frequencies, such as 900MHz or at greater frequencies, such as 10 GHz, and are generally available at 1000 watts to 3000 watts power. The present invention is not restricted to any particular microwave frequency.

In general, microwave processing systems consist of a microwave source, an applicator to deliver the power to the sample, and systems to control the heating. Microwave generators are generally vacuum tubes or solid state devices. In microwave ovens, the tubes are generally rated at 1.5 kW. The tubes need a magnetic field in order to operate, and the field is supplied by either a permanent magnet or an electromagnet. The magnetron is the most common microwave source in materials processing applications. In general, microwave energy is applied to materials through multi-mode or single-mode microwave applicators, and temperature control is typically accomplished by varying input power or through pulsed sources. The invention encompasses the use of any specialized microwave, including microwave ovens sufficient for use at home, industrial microwave ovens, and other unique microwave ovens, which may or may not use unique wavelengths for unique applications.

One significant factor limiting the potential use of microwaves for materials processing is the ability of materials to absorb microwave radiation (essentially high frequency radio waves). In contrast to conventional heating, microwaves penetrate the material with penetrating radiation. Whether or not heat is generated is determined by the specific dielectric properties of the material itself, such as the dielectric constant and dielectric loss tangent of the material.

In most materials, the microwave-power absorption is proportional to the water content of the material. However, in materials processing, unlike microwaving of food, coupling of microwave energy is to atoms or atomic groups other than water. As is commonly known, many polymeric materials are not capable of absorbing microwave radiation. Particularly, while some types of high strength fibers suitable for forming fabrics having superior ballistic penetration resistance may be capable of absorbing microwave radiation, others are not. However, it has been found that blending microwave absorbent additives with a polymeric matrix composition, or otherwise applying a microwave-reactive composition onto a material, fabrics that are otherwise not capable of absorbing microwave radiation may be suitably processed with microwave radiation. As used herein, a "microwave-reactive composition" is a composition that absorbs sufficient microwave radiation to heat a fiber or a polymeric matrix composition to at least the softening temperature of the fiber or polymeric matrix composition, respectively.

As discussed above, many polymeric materials are not capable of absorbing microwave radiation. For example, it has been found that aramid fibers are sufficiently absorbent to microwave radiation, but polyethylene fibers are not, to heat a fabric to at least the softening temperature of the fibers, or more particularly, to at least about 60°C. Particularly, Spectra® polyethylene fibers have been found to be substantially transparent to microwave radiation. Nylon, polyester and polyethylene naphthalate fibers are also at least partially microwave absorbing.

If a fabric is formed from fibers that are transparent to microwave radiation, the fibers must be at least partially coated or contacted with a material that is sufficiently microwave absorbent to reach the softening temperature of the fibers or of the polymeric matrix composition. For example, a microwave absorbent material, such as a microwave absorbent polymeric matrix composition, may be coated onto a surface of the fabric. A polymeric matrix composition may be microwave absorbent by itself, or by being blended with a microwave-reactive additive. A microwave-reactive additive will absorb the microwave energy and transfer it to the fibers. Polar materials in particular, including polar polymeric fibers, polar polymeric matrix compositions and polar additives, are microwave reactive. Conductive materials, such as conductive fibers (e.g. carbon fibers) and conductive polymeric matrix compositions (polyaniline, polypyrole, etc.) are microwave reactive. Combining non-microwave absorbent fiber types with one or more polar polymers, conductive polymers or microwave-reactive additives provides good coupling to microwave energy. Suitable microwave-reactive additives non-exclusively include metal particles, including but not limited to magnetic particles, and metal powders, dielectric particles and dielectric powders, insoluble microwave-absorbing polymeric particles and non-dispersible microwave-absorbing polymeric particles. While solid metals are known to reflect microwave radiation, powder metals do absorb microwave radiation and can be heated. Useful dielectric and polymeric powders include those that allow a polymeric matrix material to be heated to at least the softening point of the polymeric matrix material in a conventional microwave oven. Non-exclusive examples of suitable materials are described in the book Microwave Processing of Materials III, edited by Ronald L. Beatty, Willard H. Sutton and Magdy F. Iskander, published by Materials Research Society, vol. 269 (October 1992), which is incorporated herein by reference. Also suitable are the materials described in U.S. patents 5,349,168 and 6,066,375, the disclosures of which are incorporated herein by reference. Examples of useful microwave- reactive materials capable of absorbing the electric or magnetic portion of the microwave field energy and converting that energy into heat include metal powders such as powdered nickel, antimony, copper, molybdenum, bronze, iron, steel, chromium, tin, zinc, silver, gold, cobalt, tungsten, titanium, aluminum, including leafing aluminum powder, and alloys thereof. Other useful additives are conductive materials such as carbon black, carbon fibers, metal fibers, and metal flakes, spheres or needles with sizes typically ranging from about 0.1 to 100 μm. These microwave-reactive additives are particularly useful when blended with a polymeric matrix composition.

Other conductive materials such as graphite and semi-conductive materials such as silicon carbides and magnetic material such as metal oxides (if available in particulate form) may also be utilized. These materials are non-exclusive and generally any other additive material may be used that allows a polymeric matrix material or polymeric fiber to be heated to at least the softening temperature of the matrix or fiber in a conventional microwave oven at 2.45 GHz. Such materials employed are in particulate form, and may be flakes or powders. The size of such particles will vary in accordance with a number of factors, including the particular material selected, the amount of heat to be generated, the manner in which the coating composition is to be applied, and the like.

Other useful microwave-reactive additives include oils, such as watch oil, as well as glycerol, silicon carbide, calcium nitride and calcium aluminates. Other suitable additives include additives such as organic salts and inorganic salts having a high freedom of rotational, vibrational or translational movement, as well as non-conductive additives including metal oxides and metal dioxides, such as titanium dioxide, cobalt oxide, iron oxide, nickel oxide and manganese dioxide. Suitable organic salts include monosodium glutamate (MSG), potassium citrate, calcium carbonate, potassium tartrate, ammonium formate, sodium bicarbonate, maganese carbonate, and combinations thereof, as well as many others. Suitable inorganic salts include magnesium sulfate, calcium chloride, trisodium phosphate, ferrous sulphate, maganese sulphate, zinc sulphate, sodium metabisulphite, and combinations thereof, as well as many others. Also suitable are the materials disclosed in U.S. patent 4,219,361, the disclosure of which is incorporated by reference herein. These microwave-reactive additives are excited by microwaves of 2.45 GHz frequency and convert the microwave energy into thermal energy due to molecular friction. Other suitable materials that may be applied onto the fabric or blended within a matrix composition further include solutions of polyacrylate, a polyamide solution, a polyvinyl methyl ether solution, a polyamide hot melt adhesive, and a polyvinyl methyl based hot melt adhesive. It may also be suitable to soak the fabric in a processing fluid, such as water, isopropanol or ethanol, which are capable of converting at least a portion of the incident microwave energy into heat and transferring the heat to the fabric. These materials may be applied by any well known means in the art.

The amount of additive may vary depending on the type of polymeric matrix and type of additive. Typically, the microwave-reactive additive should comprise from about 0.01% to about 10.0% by weight of the polymeric matrix composition, more typically 0.01% to about 3.0% and most typically 0.01% to about 1.0% by weight of the polymeric matrix composition. Greater quantities may be used if determined to be necessary by one skilled in the art. However, in larger quantities, the additive will not stay in the matrix resin mix and will precipitate in the mixing tank when the additive is blended with the matrix resin. Depending on the temperature sensitivity of the composite's raw materials and the temperature required to achieve consolidation or reaction, both the ultimate temperature and the rate at which the composite reaches that temperature may need to be manipulated. Each specific combination would have its own preferred concentration of additive, and that concentration may vary greatly. In general, metals are more efficient than polymers, salts and other materials at reacting with and absorbing microwave radiation. Accordingly, smaller quantities of metallic based additives are typically needed.

Different substances subjected to the same amount of microwave energy heat up at different rates. For example, non-symmetrical polar molecules are easily rotated by microwave energy and heat up quickly. The principal mechanism of coupling of microwave radiation to polymers is through dipolar reorientation by an electric field. Particularly, materials having a high concentration of a strong dipole are considered to be active absorbers of microwave energy and are particularly effective. A dipole is a chemical arrangement where a positive charge and a negative charge are held at a fixed distance from each other. As a reaction of a material with microwave radiation proceeds, the type and concentration of dipole moments change, and a phase change occurs, changing the mobility of the dipoles. A dipole moment is formed by adjacent groups having different electron withdrawing/donating properties resulting in a net charge or partial charge localization on one atom or group and can be viewed as a small, weak bar magnet. Typical groups which form these dipoles include hydroxyl, amino, cyanate, etc. The efficiency of this coupling is dependent on a number of factors, including dipole strength, the mobility of the dipole and the mass of the dipole. Small strong dipoles appear to couple to microwave radiation most efficiently and liquids couple the strongest, followed by rubbers, glassy polymers and crystalline materials.

Water-based resins, as well as solvent-based and 100% solids materials, which contain dipoles will absorb microwave radiation to some extent. The strength of those dipoles, coupled with the concentration of those dipoles and the freedom of movement of the polymer, which allows the dipole to try to align itself with the oscillating magnetic field, causing friction and heat, will determine how much energy will be converted to heat. For example, a urethane linkage (-NH-COO-) is a strong dipole and polyurethane resins have a high concentration of these groups. Accordingly, polyurethane-containing matrix polymers are very effective. Polymers containing carboxylic acid groups, also a dipole, are also preferred. Other preferred polymers include poly-electrolytes, ionomers, polyvinyl alcohol, polyvinyl butyral, silicones and polyamides.

Other useful polymers that have weaker dipoles or lower concentrations of dipoles non-exclusively include acrylics, ethylene vinyl acetate and ethylene acrylic acid. These materials are active to some extent with the magnitude of the warming related to the strength and concentration of the dipoles. Also suitable are blends of matrix polymers, such as a two-phase matrix of an active resin dispersed in a non-active resin, allowing the non-active resin to be processed with microwave energy.

The selection of the most effective microwave-reactive additive is generally dependent upon the frequency, power and duration of the microwave energy to be absorbed. It is known that heating is accelerated by ionic effects and the specific heat of a composite material. For example, oils are useful materials because of their low specific heat. Many factors also contribute to the minimum amount of microwave-reactive additive required. Highly active materials will generally require a lower weight or volume percentage. Synergistic compositions (some absorbed free water, induced dipoles in other constituents of the composite, etc.) will reduce the required level of active component. Lower targeted process temperatures would also require lower levels of highly active component, or higher levels of a lower absorbing material. Typically, the minimum amount of microwave-reactive additive required will be less than about 10% by weight of the fabric, more preferably, less than 10% by weight of the polymeric matrix composition. More preferably, the quantity of microwave-reactive additive will be from about 1% to about 6% by weight, more preferably from about 3% to about 6% by weight of the polymeric matrix composition, or by weight of the fabric if no matrix is present. If the microwave-reactive additive is dispersed as a mixture in a solvent, mixture will typically include about 70% to about 80% by weight of the solvent.

In the method of the invention, a fabric is heated inside a microwave oven by subjecting the fabric to microwave energy sufficient to thereby heat the fibers or the optional microwave-reactive composition to at least about the softening temperature of the fibers or of the optional microwave-reactive composition. Prior to microwave heating, it is preferred that the ballistic resistant fabric and optional microwave-reactive composition be completely dry and free of volatile substances. The material should be heated to a temperature less than a temperature at which the material is degraded or burned. Immediately after heating or during heating, the fabric is molded or consolidated into an article while said fabric has a temperature of at least about the softening temperature of the fibers or the softening temperature of the optional microwave-reactive composition, if present. As used herein, the term "immediately thereafter" means that the fabric is molded or consolidated while still at or above the softening temperature due to microwave generated heat. Thereafter, the heated fabric is molded into an article while said fabric has a temperature of at least about the softening temperature of the fibers or the softening temperature of the optional microwave-reactive composition, if present. Alternately, the fabric may be heated and molded consecutively in a single multifunctional apparatus having both heating and molding capabilities. While it is envisioned that the optional transfer of the heated fabric from the microwave to a separate molding apparatus might cause the fabric to lose some heat, the process is conducted such that molding is commenced while the fabric retains sufficient microwave generated heat to allow the fabric to be molded into any desired shape or form, and allowing the fabric to retain said shape if so intended. Finally, the molded fabric is allowed to cool.

As described herein, a fabric of the invention must be heated until it reaches a temperature suitable for molding. The minimum molding temperature of a fabric is typically determined by the softening temperature point of either the polymeric matrix composition or the softening temperature point of the fibers if no matrix composition is present. As is commonly known in the art, the softening point of plastics may be measured by the ASTM Dl 525 Vicat Softening Temperature testing method, which covers determination of the temperature at which a specified needle penetration occurs when specimens are subjected to specified controlled test conditions. More particularly, in this testing method, a flat-ended needle loaded with a specified mass is placed in direct contact with a test specimen. The specimen and needle are heated at a permissible rate, and the temperature at which the needle has penetrated to a depth of 1 ± 0.01 mm is recorded as Vicat softening temperature.

Suitable minimum molding temperatures typically range from about 60⁰C to about 180⁰C, but vary depending on the particular fiber type, and may be beyond this range. For example, Spectra® polyethylene fibers are affected after prolonged exposure to heat above 265°F (129.4°C) and melt at 300⁰F (148.9°C). Accordingly, Spectra® polyethylene fibers are preferably heated to greater than about 200⁰F (93.3°C) but less than about 257°F (125°C). When heating by convection, the heating step commonly adds an additional 10 to 30 minutes to the fabric processing time and requires pre-heating of the convection oven. This heating time is substantially reduced by microwaving the fabric. The exposure time to microwave energy should be enough to sufficiently heat the fabric to the desired temperature, while brief enough to avoid degradation of the fibers. Most preferably, the fabric is capable of being heated in a microwave oven to 200⁰F or greater within three minutes.

The complete molded fabrics of the invention comprise a combination of fibers, an optional matrix composition, optional intermediate adhesive layers and an optional microwave sensitive material. In general, to produce a fabric having sufficient ballistic resistance properties, the proportion of fibers preferably comprises from about 45% by weight to about 95% by weight of the fabric, more preferably from about 60% to about 90% by weight of the fabric, and most preferably from about 65 to about 85% by weight of the composite. As is commonly known in the art, the matrix composition and/or optional adhesive may also include other additives such as fillers, such as carbon black or silica, may be extended with oils, or may be vulcanized by sulfur, peroxide, metal oxide or radiation cure systems as is well known in the art.

Depending on the desired structural and ballistic resistance properties of the articles formed from the fabrics of the invention, various parameters such as the number and type of fabric layers and the type of matrix may be controlled. For example, for the formation of low cost trauma pads for reducing deformation in ballistic resistant vests, it is preferred to include 2 fabric layers, i.e. two woven fibrous layers, or two single layer, consolidated networks of non- woven, unidirectional fibers, each formed from two fiber layer plies cross-plied at 0°/90°, having a rubber layer on either outer surface of the combined fabric. Further, for ballistic resistant panels having a ballistic protection level of NIJ Level II or IIA, fabrics including 14 fabric layers and 10 fabric layers, respectively, are preferred.

Whether the component fibers of a fabric are capable of absorbing microwave radiation or whether a microwave-reactive composition is required, the fabrics of the invention are capable of being heated to a temperature of at least about 60⁰C by microwave radiation in a microwave oven. The microwave oven may operate at any frequency and at any microwave power setting. Most preferably, fabrics of the invention are capable of being heated in a microwave oven to 200°F (93.3⁰C) or greater within three minutes.

The following non-limiting examples serve to illustrate the invention. EXAMPLES 1-6 (COMPARATIVE)

Microwave heating trials were conducted on Spectra Shield® ("SS") non-woven fabric samples formed with Spectra® fibers (1300-denier, type 1000), and KRATON® styrene-isoprene-styrene (SIS) polymeric matrix resin (KRATON® D-1161 : 40 wt. % supplied solids, diluted to 16 wt. % solids content applied onto fabric) or SANCURE® 12929 polyurethane matrix resin (commercially available from Noveon, Inc. of Cleveland, Ohio, a subsidiary of the Lubrizol Corporation). In each case, the samples were made with 20±2 wt. % resin content, and had a non-woven, cross-plied Spectra Shield® material (0°, 90° construction).

Ten test coupons (2" x 2") (5.08 cm x 5.08 cm) each including two 0790° cross- plied and consolidated plies, were cut from each sample, stacked and exposed to different levels of microwave energy for various durations using a standard 2.45 GHz, 1500 watt home microwave oven. The temperature of the fabric upon heating was measured. The results are summarized in Table IA.

TABLE IA

In each of Examples 1-6, the KRATON® polymeric matrix material failed to reach a temperature of about 113 ⁰F when subjected to microwave radiation at the specified conditions, much lower than the softening point of the KRATON® polymer. Accordingly, KRATON® polymer alone is insufficiently microwave absorbent to generate the minimum amount of heat required for molding of Spectra Shield® material.

EXAMPLES 7-10 Microwave heating trials were conducted on Spectra Shield® ("SS") material, non- woven fabric samples formed with Spectra® fibers (1300-denier, type 1000) and SANCURE® 12929 polyurethane matrix resin (commercially available from Noveon, Inc. of Cleveland, Ohio, a subsidiary of the Lubrizol Corporation). In each case, the samples were made with 20±2 wt. % resin content, and had a non- woven, cross-plied Spectra Shield® (0°,90°) construction.

Ten test coupons (2" x 2") (5.08 cm x 5.08 cm) each including two 0°/90° cross- plied and consolidated plies, were cut from each sample, stacked and exposed to different levels of microwave energy for various durations using a standard 2.45 GHz, 1500 watt home microwave oven. The temperature of the fabric upon heating was measured. The results are summarized in Table IB.

TABLE IB

The above Examples show that Sancure® 12929 polymeric matrix resin provides the microwave heating capability to Spectra Shield® for pre-heating. EXAMPLES 11-16 (COMPARATIVE)

Similar to Examples 1-10, microwave heating trials were conducted on Spectra Shield® non- woven fabric samples formed with Spectra® fiber (1300-denier, type 1000) and Good-Rite® SB-1168 styrene-butadiene-styrene (SBS) rubber polymeric matrix resin (commercially available from Noveon, Inc. of Cleveland, Ohio). In each case, samples were made with 20±2 wt. % resin content, and had a non-woven, cross-plied Spectra Shield® (0°, 90°) construction.

Ten test coupons (2" x 2") (5.08 cm x 5.08 cm) each including two 0°/90° cross- plied and consolidated plies, were cut from each sample, stacked and exposed to different levels of microwave energy for various durations using a standard 2.45 GHz, 1500 watt home microwave oven. The temperature of the fabric upon heating was measured. The results are summarized in Table 2.

TABLE 2

The above Examples show that Good-Rite® SB-1168 SBS Rubber polymeric matrix resin does not provide a microwave heating capability to Spectra Shield® for pre-heating.

EXAMPLES 17-32

Similar to Examples 1-16, microwave heating trials were conducted on Spectra Shield® non- woven fabric samples formed with Spectra® fiber (1300-denier, type 1000) and various polymeric matrix polymers. In each case, samples were made with 20±2 wt. % resin content, and had a non-woven, cross-plied Spectra Shield® material (0°, 90° construction).

The tested polymeric matrix polymers were:

Example 17: Airflex® 4500, an amide-modified ethylene-vinyl chloride copolymer available from Air Products and Chemicals, Inc. Example 18: Permax™ 230, a polyurethane resin available from Noveon, Inc.

Example 19: Hycar® 26523, an acrylic available from Noveon, Inc.

Example 20: Hycar® 26-1475, an acrylic available from Noveon, Inc.

Example 21 : Hycar® 26-1199, an acrylic available from Noveon, Inc.

Example 22: Sancure® 20023, a polyurethane resin available from Noveon, Inc.

Example 23: Good-Rite® SB-1168, a carboxyl -modified styrene-butadiene- styrene copolymer available from Noveon, Inc.

Example 24: Daran® SLl 12, a PVdC polymer available from W. R. Grace & Co.

Example 25: Permax™ 803, an acrylic-PVdC copolymer available from Noveon, Inc.

Example 26: Sancure® 777, a polyurethane resin available from Noveon, Inc.

Example 27: Sancure® 843, a polyurethane resin available from Noveon, Inc.

Example 28: Dispercoll® U53, a polyurethane resin available from Bayer AG. Example 29: Vycar® 460X251 , a PVC copolymer available from Noveon, Inc. Example 30: Sancure® 20025, a polyurethane resin available from Noveon, Inc. Example 31 : Butvar® RS-261 , a polyvinylbutyral available from Solutia, Inc. Example 32: Sancure® 2026, a polyurethane resin available from Noveon, Inc.

Ten test coupons (2" x 2") (5.08 cm x 5.08 cm) each including two 0°/90° cross- plied and consolidated plies, were cut from each sample, stacked and exposed to microwave energy for 60 seconds using a standard 2.45 GHz home microwave oven at 50% power. The highest fabric temperature after being microwaved for one minute was measured, the temperature reached after one minute in a microwave following forced drying in a conventional oven was measured, and the time to reach 175°F in a 2.45 GHz microwave oven at 50% power was estimated for each sample. The results are summarized in Table 3. In column 4, rows with a dash indicates that the highest fabric temperature after 1 minute in the microwave was not measured.

For each of the examples, prior to subjecting samples to microwave radiation, the samples were heated in an oven to remove any water or other volatile components in the resin dispersion. The samples were initially dried in an oven at 150⁰F (65.56°C) for five days. Once microwave testing commenced, some samples popped indicating the presence of residual water or other volatiles. These samples were then placed back into the oven for another five days at 200⁰F (93.33 ⁰C) to complete the removal of any water and/or volatiles.

For each of the examples, the response to microwave radiation was evaluated according to the following procedures:

1. A circular 1" (2.54 cm) thick section of ST YROFO AM™ was placed onto the carousel of a 1500 watt residential-use microwave oven. This STYROFOAM™ was used to isolate any heat generated by the sample under evaluation from any heat generated by the ceramic carousel plate.

2. Four samples of the material being evaluated were placed onto the STYROFOAM™ section. These samples were placed towards the edge of the STYROFOAM™ at the 12:00, 3:00, 6:00 and 9:00 positions. The four samples were spaced 3" (7.62 cm) from each other.

3. Next, Tempilstik® temperature indicators, manufactured by Illinois Tool Works Inc. of Illinois, were used to evaluate temperature thresholds.

4. The desired temperature ranges were tested using two Tempilstik crayons with temperature ratings below the targeted temperature and two crayons with activation ranges above the targeted temperature.

5. Using an indelible pen, each of the four samples of consolidated fabric were marked with one of the four temperatures. Shavings were scraped from one of the crayons onto the fabric sample that has the appropriate temperature writing on its surface. This was also done with the other three crayons and the other three samples.

6. The microwave oven was closed, set to the desired power level, a duration time was set and the microwave heating was initiated.

7. Thereafter, it was determined which of the waxes melted onto the surface of the fabric sample.

TABLE 3

All of the polymeric matrix resins tested in Examples 7-38 were water-based resin dispersions. Some were successful and others were unsuccessful at absorbing microwave radiation. The Kraton® D-1161 resin tested in Examples 1-6 was a solvent-based resin and was unsuccessful at absorbing microwave radiation. However, it is expected that other solvent-based resins would be successful, and it is not expected that an aqueous polymeric matrix composition is required.

While the present invention has been particularly shown and described with reference to preferred embodiments, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended that the claims be interpreted to cover the disclosed embodiment, those alternatives which have been discussed above and all equivalents thereto.

Claims

What is claimed is:

1. A method of forming an article comprising: a) providing a fabric comprising a plurality of fibers arranged in an array, said fibers having a tenacity of about 7 g/denier or more and a tensile modulus of about 150 g/denier or more; said fibers having an optional microwave-reactive composition thereon; and b) heating said fabric inside a microwave oven by subjecting the fabric to microwave energy sufficient to thereby heat the fibers or the optional microwave- reactive composition to at least about the softening temperature of the fibers or the softening temperature of the optional microwave-reactive composition; c) molding the heated fabric into an article while said fabric has a temperature of at least about the softening temperature of the fibers or the softening temperature of the optional microwave-reactive composition due to the application of microwave energy; and d) allowing the molded fabric to cool.

2. The method of claim 1 wherein said microwave-reactive composition is present and the fabric is heated by subjecting the fabric to microwave energy sufficient to thereby heat the microwave-reactive composition to at least about its softening temperature; and molding the heated fabric into an article while said fabric has a temperature of at least about the softening temperature of the microwave-reactive composition.

3. The method of claim 2 wherein said microwave-reactive composition comprises a polymeric matrix composition which is coated on said fibers.

4. The method of claim 1 wherein said fabric is heated to a temperature of at least about 60⁰C.

5. The method of claim 1 wherein said fabric comprises a plurality of fibers wherein one or more of said fibers are capable of absorbing sufficient microwave radiation to heat said fiber to a temperature of at least about 60°C; wherein the fabric is heated by subjecting the fabric to microwave energy sufficient to thereby heat the fibers to at least about the softening temperature of the fibers; and immediately thereafter molding the heated fabric into an article while said fabric has a temperature of at least about the softening temperature of the fibers.

6. The method of claim 1 wherein said microwave-reactive composition comprises a dipole containing polymer.

7. The method of claim 1 wherein said microwave-reactive composition comprises a polyurethane, poly-electrolyte, an ionomer, polyvinyl alcohol, polyvinyl butyral, a silicone, a polyamide, an acrylic, ethylene vinyl acetate, ethylene acrylic acid, or a combination thereof.

8. The method of claim 1 wherein said microwave-reactive composition comprises a polymer containing a microwave-reactive additive, which microwave-reactive additive is capable of absorbing sufficient microwave radiation to heat said fiber to a temperature of at least about 60⁰C.

9. The method of claim 8 wherein said microwave-reactive additive comprises a polar composition.

10. The method of claim 8 wherein said microwave-reactive additive comprises an organic salt, an inorganic salt, a metallic powder, a dielectric powder, an insoluble microwave-absorbing polymeric powder, a non-dispersible microwave- absorbing polymeric powder, or a combination thereof.

11. The method of claim 1 wherein said plurality of fibers comprise polyolefin fibers, aramid fibers, polybenzazole fibers, polyvinyl alcohol fibers, polyamide fibers, polyethylene terephthalate fibers, polyethylene naphthalate fibers, polyacrylonitrile fibers, liquid crystal copolyester fibers, glass fibers, carbon fibers, rigid rod fibers, or a combination thereof.

12. The method of claim 1 wherein said fabric comprises a plurality of polyethylene fibers, wherein one or more of said polyethylene fibers have a microwave-reactive composition thereon wherein said microwave-reactive composition is capable of absorbing sufficient microwave radiation to heat said fiber to a temperature of at least about 60⁰C.

13. The method of claim 1 wherein said heating step comprises subjecting the fabric to microwave energy at a frequency of at least about 2.45 GHz.

14. The method of claim 1 wherein said heating step comprises subjecting the fabric to microwave energy at a frequency of at least about 10 GHz.

15. The method of claim 1 wherein said molding step c) is conducted after said heating step b).

16. The method of claim 1 wherein said molding step c) is conducted consecutively with said heating step b).

17. A method of forming a consolidated fiber network, said consolidated network of fibers comprising a plurality of fiber layers, each fiber layer comprising a plurality of fibers having a tenacity of about 7 g/denier or more and a tensile modulus of about 150 g/denier or more; and said fibers having a polymeric matrix composition thereon; which consolidated fiber network is consolidated under heat and pressure, wherein the heat of consolidation is generated by the application of microwave energy sufficient to thereby heat the polymeric matrix composition to a temperature of at least about the softening temperature of polymeric matrix composition.

18. The method of claim 17 wherein the softening temperature of polymeric matrix composition is at least about 6O⁰C as measured by ASTM D 1525.

19. The method of claim 17 wherein said consolidated fiber network comprises a plurality of cross-plied, non- woven fiber layers, each fiber layer comprising a plurality of fibers arranged in a substantially parallel array.

20. A ballistic resistant article comprising a ballistic resistant fabric, the ballistic resistant fabric comprising a plurality of fibers arranged in an array, said fibers having a tenacity of about 7 g/denier or more and a tensile modulus of about 150 g/denier or more; said fibers having a dry, microwave-reactive composition coated thereon, which microwave-reactive composition has been heated above its softening point temperature by the application of microwave energy.

21. The ballistic resistant article of claim 20 which has been has been heated above its softening point temperature by the application of microwave energy and molded into an article while at a temperature of at least about the softening temperature of the fibers or the softening temperature of the optional microwave- reactive composition due to the application of microwave energy.

22. An article produced by the method of claim 1.