Ballistic resistant composite armour

Abstract: A rigid improved penetration resistant composite of the type comprising a plurality of fibrous layers comprising network of fibers in a polymeric matrix selected from the group consisting of thermoplastic polymers, thermosetting resins or a combination thereof, at least two of said layers secured together by a securing means, said improvement comprising a securing means which comprises fiber stitches wherein the average stitch length is greater than the average stitch path, a process for forming the composite and articles fabricated from said composite.

Description: This invention relates to articles having improved resistance to forces as for example penetration by ballistic projectiles, pointed objects (e.g. knives, icepicks, etc.) and the like and the effect of explosive blast. More particularly, this invention relates to such articles which are fiber based and which are especially suitable for fabrication into penetration and blast resistant articles such as ballistic armor, as for example, helmets, shields which are resistant to impact by ballistic projectiles, portable barricades and armor panel inserts for briefcases, spall liners for military vehicles personnel carriers and tanks, aircraft armor, seats in military vehicles, panels for explosives containers and explosion resistant cargo containers for aircraft and the like. Ballistic articles such as helmets, structural members of helicopters and other military equipment, and vehicle panels, containing high strength fibers are known. Illustrative of such articles are those described in US-A- 3,971,072 ; 3,988,780 ; 4,183,097; 3,855,632 ; 4,522,871 ; 4,510,200 ; 4,623,574; 4,748,064; 4,916,000; 4,403,012; 4,457,985; 4,650,710; 4,681,792; 4,737,401; 4,543,286; 4,563,392; and 4,501,856. US-A-3841954 discloses compressed rigid laminated material primarily intended as a backing for personal armour having a hard surface. The material comprises multiple layers of initially flexible material including stitching reinforcement with a heat shrunk filament, the layers of the flexible material having been compressed and made rigid, reinforcing the material, by the heat shrinking of the stitching. US-A-3971072 provides reinforced body armor having a thin ballistic metal outer shell secured to a plurality of layers of flexible ballistic material. The layers of material are sewn together along paths spaced within a selected predetermined range, so as to restrict movement of the fabric layers in lateral and longitudinal directions and to compact the layers in an elastic mass thereby to provide improved resistance to penetration of the material by a ballistic missile and to partially stiffen the material so that shock waves and the force of impact of the missile are distributed over a relatively large area adjacent the point of impact of the missile to reduce back target distortion. The present invention relates to a rigid penetration resistant composite comprising a plurality of fibrous layers, said fibrous layers being comprised of a network of fibers dispersed in a polymeric matrix, at least two of said layers being secured together by a plurality of stitches preferably extending along at least two spaced paths, which spaced paths are preferably adjacent or substantially adjacent paths, said stitches having a stitch length and a stitch path as defined below, wherein all or substantially all of said stitches have a stitch length which is greater than the length of the stitch path. Another embodiment of this invention relates to an article of manufacture comprising a body which is constructed totally or in part from the composite of this invention. As used herein, the "penetration resistance" of the article is the resistance to penetration by a designated threat. Such designated threats include physical objects as for example, bullets, fragments, shrapnels and the like, threats also include non-physical objects such as blast from explosion and the like. The penetration resistance for designated threats can be expressed by at least three methods: 1. V/50 is the velocity at which 50% of the threats will penetrate the composite while 50% will be stopped by the armor. For composite of equal areal density, which is the weight of the composite panel divided by the surface area, the higher the V/50, the better the resistance of the composite; 2. Total specific energy absorption (SEAT): SEAT is the kinetic energy of the threat divided by the areal density of the composite. The higher the SEAT value, the better the resistance of the composite to the threat; and 3. Striking velocity (V/s) vs. residual velocity (V/r): When a threat strikes an armor panel at a velocity of (V/s), the residual velocity (V/r) is measured after the threat penetrates the composite. The larger the difference between (V/s) and (V/r), the better the resistance to the threat for composite panels of equal areal density. In ballistic studies, the specific weight of the composites can be expressed in terms of the areal density (ADT). This areal density corresponds to the weight per unit area of the ballistic resistant armor. In the case of filament reinforced composites,- the ballistic resistance of which depends mostly on filaments, another useful weight characteristic is the filament areal density of the composite. This term corresponds to the weight of the filament reinforcement per unit area of the composite (AD). As used herein the "stitch length" is the cycle length of the fiber from the exit from one surface of the composite (the first surface) to the next exit of the fiber from the same surface of the composite, and is the combination of the portion of the stitch from the exit of the stitch fiber on to a surface of the composite along the surface of the composite to the entrance of the stitch into the composite, the portion of the stitch fiber passing through the composite to the exit of the stitch fiber onto the opposite surface of the composite, the portion of the stitch fiber from the exit on the opposite surface of the composite along the opposite surface of the composite and the portion of the stitch that passes through the composite from the opposite surface to the exit of the stitch on said first surface; and the "length of the stitch path" is the linear distance traversed by the stitch from an exit of the stitch from a surface of the composite to the next subsequent exit of the stitch fiber from said surface of the composite and is the sum of twice the thickness of the composite and the distance on the surface of the composite between the exit of the stitch fiber from a surface of the composites and the following entry of the stitch fiber into the surface of the composite. As used herein, "rigid" means that the composite is not flexible which is measured by clamping a 30 cm. composite horizontally along an edge with an overhang of 20 cm. and measuring the amount of drape of the composite and then rotating the composite by 90° and by 180° and again measuring the amount of drape of the composite at 90° and 180°. The amount of drape is measured by the vertical distance between the level of the clamped side edge, and the level of the opposite edge : where the drape is about 0 cm. the composite is rigid. Several advantages flow from this invention. For example, the articles and composite of this invention exhibit relatively improved penetration resistance as compared to other articles and composites of the same construction and composition but having stitch lengths and stitch paths outside the scope of this invention. The relationship between the stitch length and the stitch path controls the delamination of the composite to achieve optimum ballistic performance and to prevent separation of the composite due to impact. It has been discovered that controlled level of delamination is advantageous to penetration resistance because delamination consumes part of the total kinetic energy of the threat and less of the total energy is available to penetrate the composite, resulting in superior performance. Another advantage resulting from the relationship between the stitch length and the stitch path is improved multiple hit capability. Other advantages of the articles and composites of this invention will become apparent from the specification and claims. This invention also relates to a process for fabricating the penetration resistant composite of this invention comprising the steps of: a) stitching a plurality of adjacent fibrous layers, each of which comprises a network of fibers dispersed in a polymeric matrix, by a plurality of stitches having an average first stitch length and having an average first -stitch path, wherein said first stitch length is equal to or substantially equal to said first stitch path to form a first composite having a first thickness, t 1 ; and b) compressing said first composite at a second temperature and or second pressure to form a penetration resistant second composite having a second thickness, t 2 , a second stitch length and a second stitch path wherein said first thickness, t 1 , is greater than said second thickness, t 2 , said second stitch length is equal to or substantially equal to said first stitch length and wherein said second stitch path is less than said first stitch path. The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the invention and the accompanying drawings in which: FIG. 1 is a fragmentary view of a preferred composite of this invention in which certain selected layers have been cut away. FIG. 2 is an exploded fragmentary view of the preferred embodiment of this invention depicted in FIG. 1 in which an exploded view of stitch length and stitch paths are depicted. FIG. 3 is an exploded fragmentary view of another preferred embodiment of this invention in which an exploded view of stitch length and stitch paths are depicted. FIG. 4 is a fragmentary view of a precursor composite of the composite of FIG. 1. The preferred embodiments of this invention will be better understood by those of skill in the art by reference to the above figures. The preferred embodiments of this invention illustrated in the figures are not intended to be exhaustive or to limit the invention to the precise form disclosed. They are chosen to describe or to best explain the principles of the invention and its application and practical use to thereby enable others skilled in the art to best utilize the invention. Referring to FIGs. 1 and 2, the numeral 10 indicates a penetration or blast resistant composite 10, which in this preferred embodiment of the invention is a penetration resistant body armor which comprises a plurality of resin impregnated fibrous layers 12a to 12j which are stitched together by a plurality of stitches 14 and 16. As shown in FIG. 2, stitches 14 and 16 have an average stitch length defined by the actual length of the stitch fiber from point "a" to point "e", and an average stitch path defined by the linear distance from point "a" to point "e". The relationship between the stitch path and the stitch length is critical to the advantages of this invention. In general, while the exact values of the stitch length and the stitch path may vary widely the stitch length must be greater than the stitch path. All stitches need not have the same or substantially the same stitch length or stitch path. The only requirement is that all or substantially all stitches have stitch paths and stitch lengths having the stated relationship. In the preferred embodiments of the invention, all or substantially all of the stitches have equal or substantially equal stitch lengths and equal or substantially equal stitch paths, where for any stitch, the stitch path and stitch length have the stated relationship. Usually, the stitch length is up to 300% of the stitch path. In the preferred embodiments of the invention the stitch length is from 102 to 300% of the stitch path. The stitch length is more preferably from 105 to 200% of the stitch path and is most preferably from 105 to 150% of the length of the stitch path. In the embodiments of this invention of choice, the stitch length is from 110% to 150% of the length of the stitch path. The actual values of the stitch length and stitch path are not critical and may vary widely and depending on the use of the composite and desires of the fabricator provided that they have the required relationship. In most instances where the composite is for use in penetration or blast resistant applications, the stitch length and the length of the stitch path is equal to or less than 11.4 cm. The stitch length is preferably from 11.4 cm to 1 cm and the stitch path is preferably from 8.9 to 0.64 cm; the stitch length is more preferably from 9.5 to 1 cm and the stitch path is more preferably from 7.3 to 0.64 cm; and the stitch length is most preferably from 7.7 to 1.5 cm. the stitch path is most preferably from 6.9 to 0.96 cm. The angle of the stitch path is not critical and may vary widely. As used herein, the "angle of the stitch path" is formed between two diverging lines drawn from a common point on the composite exterior surfaces where one line is the shortest distance drawn perpendicularly through the composite while the other line is the actual stitch path. Generally, the angle of the stitch path is equal to or less than 90°. The angle of the stitch path is preferably from 0° to 60°, more preferably from 0° to 45° and most preferably from 0° to 30°. The amount of stitches employed may vary widely. In general in pentration resistance applications, the amount of stitches employed is such that the stitches comprise less than 10% of the total weight of the stitched fibrous layers. The weight percent of stitches is preferably from 0.01 to 10, more preferably from 0.02 to 5 and most preferably from 0.05 to 1, on the aforementioned basis. Composite 10 may include stitches extending along a single path, or stitches extending along more than one path, which may be parallel or substantially parallel or which may intersect at an angle, or a combination thereof. In the preferred embodiments of this invention, composite 10 includes stitches which extend along more than one path which are parallel or substantially parallel. An example of this embodiment of the invention can be represented by FIG.1 which includes parallel or substantially parallel stitches 16 and parallel or substantially parallel stitches 14. In this preferred embodiments of the invention, distance between parallel or substantially parallel stitch paths may vary widely. For penetration resistance applications such distance is generally equal to or less than 10 cm, preferably from 0.254 to 8 cm, more preferably from 0.5 to 5 cm and most preferably from 0.5 to 3 cm. In another preferred embodiment of this invention depicted in FIG.1, composite 10 includes two sets of stitches, parallel or substantially parallel stitches 14 and parallel or substantially parallel stitches 16 which intersect at an angle. The angle of intersect may vary widely. In general for penetration resistant applications, the angle of intersection is from 60° to 150°, more preferably from 75° to 135°, and most preferably from 85° to 95°. As depicted in FIG 1 , article 10 is comprised of ten layers 12a to 12j. However, the number of layers 12 included in article 10 may vary widely, provided that at least two layers are present. In general, the number of layers in any embodiment will vary depending on the degree of penetration resistance desired. In general, the greater the number of layers 12 the greater the penetration resistance of the composite, and the lesser the number of layers 12 the lower the penetration resistance of the composite.The number of fibrous layers 12 is preferably from 2 to 1500, more preferably from 10 to 1400 and most preferably from 40 to 1000. The type of stitches employed is not critical and may vary widely provided that the required relationship between stitch length and stitch path is provided. Stitching and sewing methods, such as hand stitching , multi-thread chain stitching, over edge stitching, flat seam stitching, single thread lock stitching, lock stitching, chain stitching, zig-zag stitching and the like constitute the preferred securing means for use in this invention. The fiber used to form stitches 14 and 16 in these preferred embodiments can vary widely. Useful fiber may have a relatively low modulus or a relatively high modulus and may have a relatively low tenacity or a relatively high tenacity. Fiber for use in stitches 14 and 16 preferably has a modulus equal to or greater than 20 grams/denier (17.7 dN/tex) and a tenacity equal to or greater than 2 grams/denier (1.8 dN/tex). All tensile properties are evaluated by pulling at 10 in (25.4cm) fiber length clamped barrel clamps at 10 in/min (25.4cm/min) on an Instron Tensile Tester. In the preferred embodiments of the invention, the modulus is equal or greater than 30 grams/denier (26.5 dN/tex) and the tenacity is equal to or greater than 4 grams /denier (3.5 dN/tex), preferably from 6 to 50 grams/denier (5.3 to 44.2 dN/tex), more preferably the modulus is from 40 to 3000 grams/denier (35.3 to 2649 dN/tex) and the tenacity is from 8 to 50 grams/denier (7.1 to 44.2 dN/tex) and most preferably the modulus is from 300 to 3000 grams/denier (264.9 to 2649 dN/tex) and the tenacity is from 10 to 50 grams/denier (8.8 to 44.2 dN/tex). Useful threads and fibers may vary widely and will be described in more detail herein below in the discussion of fiber for use in the fabrication of fibrous layers 12. Useful fibers may be formed from inorganic materials as for example graphite, boron, silicon nitride, silicon carbide, glass (e.g. S-glass and E-glass) and the like. Useful fibers may also be formed from organic materials as for example thermosetting and thermoplastic polymer. However, the thread or fiber used in stitching means is preferably formed from an organic material ,more preferably a polymeric material and most preferably an aramid fiber or thread, an extended chain polyethylene thread or fiber, a nylon (e.g. nylon 6, nylon 11, nylon 6,10 and nylon 6,6) thread or fiber, liquid crystalline copolyester thread or fiber, or mixtures thereof. Fibrous layer 12 comprises a network of fibers in a polymeric matrix. For purposes of the present invention, fiber is defined as an elongated body, the length dimension of which is much greater than the dimensions of width and thickness. Accordingly, the term fiber as used herein includes a monofilament elongated body, a multifilament elongated body, ribbon, strip and the like having regular or irregular cross sections. The term fibers includes a plurality of any one or combination of the above. The cross-section of fibers for use in this invention may vary widely. Useful fibers may have a circular cross-section, oblong cross-section or 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. In the particularly preferred embodiments of the invention, the fibers are of substantially circular or oblong cross-section and in the most preferred embodiments are of circular or substantially circular cross-section. The type of fiber used in the fabrication of fibrous layer 12 may vary widely and is preferably an organic fiber. Preferred fibers for use in the practice of this invention are those having a tenacity equal to or greater than 7 grams/denier (g/d) (6.2 dN/tex), a tensile modulus equal to or greater than 50 g/d (44.2 dN/tex) and an energy-to-break equal to or greater than 30 joules/gram. The tensile properties are determined by an Instron Tensile Tester by pulling the fiber at 10 in (25.4cm) fiber length, clamped in barrel clamps at 10 in/min (25.4cm/min). Among these particularly preferred embodiments, most preferred are those embodiments in which the tenacity of the fiber are equal to or greater than 15 g/d (13.2 dN/tex), the tensile modulus is equal to or greater than 300 g/d (264.9 dN/tex), and the energy-to-break is equal to or greater than 20 joules/gram. In the practice of this invention, fiber of choice have a tenacity equal to or greater than 20 g/d (17.7 dN/tex), the tensile modulus is equal to or greater than 1300 g/d (1147.9 dN/tex) and the energy-to-break is equal to or greater than 40 joules/gram. The denier of the fiber may vary widely. In general, fiber denier is equal to or less than 20,000 (2222 tex). In the preferred embodiments of the invention, fiber denier is from 10 to 20,000 (1.1 to 2222 tex), the more preferred embodiments of the invention fiber denier is from 10 to 10,000 (1.1 to 1111 tex) and in the most preferred embodiments of the invention, fiber denier is from 100 to 10,000 (11.1 to 1111 tex). Preferably, yarn or fibers for use in the invention consists of multi-ends of filaments. The denier of each filament preferably varies from 1 to 25 denier (0.1 to 2.8 tex). Illustrative of useful organic fiber are those composed of thermosetting resins and thermoplastic polymers such as polyesters ; polyolefins ; polyetheramides; fluoropolymers; polyethers; celluloses; phenolics; polyesteramides; polyurethanes; epoxies; aminoplastics; polysulfones; polyetherketones; polyetherether-ketones; polyesterimides; polyphenylene sulfides; polyether acryl ketones; poly(amideimides); polyimides; aramids (aromatic polyamides), such as poly(2,2,2-trimethyl-hexamethylene terephthalamide) (Kevlar)and the like; aliphatic and cycloaliphatic polyamides, such as polyhexamethylene adipamide (nylon 66), polycaprolactam (nylon 6)and the like; and aliphatic, cycloaliphatic and aromatic polyesters such as poly (1,4-cyclohexlidene dimethylene terephathalate) cis and trans, , poly(ethylene terephthalate) and the like. Also illustrative of useful organic filaments are those of liquid crystalline polymers such as lyotropic liquid crystalline polymers which include polypeptides such as poly-benzyl L-glutamate and the like; aromatic polyamides such as poly(1,4-benzamide), poly(4,4'-biphenylene 4,4'-bibenzo amide), poly(1,4-phenylene 4,4'-terephenylene amide), poly(1,4-phenylene 2,6-naphthal amide), and the like; polyoxamides such as those derived from 2,2' dimethyl-4,4'diamino biphenyl, chloro-1,4-phenylene diamine and the like; polyhydrazides such as poly chloroterephthalic hydrazide and the like;poly(amide hydrazides such as poly(terephthaloyl 1,4 amino-benzhydrazide) and those prepared from 4-amino-benzhydrazide, oxalic dihydrazide, terephthalic dihydrazide and para-aromatic diacid chlorides; polyesters such as poly(oxy-trans-1,4-cyclohexyleneoxycarbonyl-trans-1,4--cyclohexylenecarbonyl-trans-1,4phenyleneoxyterephthaloyl), poly(oxy-trans-1,4-cyclohexylene-oxycarbonyl-trans-1,-4-cyclohexylenecarbonyl-b-oxy-(2-methyl 1,4-phenylene)oxy-terephthaloyl)],and the like; polyazomethines such as those prepared from 4,4'-diaminobenzanilide and terephthalaldephide, methyl-1,4-phenylenediamine and terephthalaldelyde and the like; polyisocyanides such as poly(-phenyl ethyl isocyanide), poly(n-octyl isocyanide) and the like; polyisocyanates such as poly (n-alkyl isocyanates) as for example poly(n-butyl isocyanate), poly(n-hexyl isocyanate) and the like; lyotropic crystalline polymers with heterocyclic units such as poly(1,4-phenylene-2,6-benzobisoxazole) (PBO), poly(1,4-phenylene-1,3,4-oxadiazole), poly(1,4-phenylene-2,6-benzobisimidazole), poly[2,5(6)-benzimidazole] (AB-PBI), poly[2,6-(1,4-phenylene)-4-phenylquinoline], poly[1,1'-biphenylene)-6,6'-bis(4-phenylquinoline)] and the like; polyorganophosphazines such as polyphosphazine, polybisphenoxyphosphazine, poly]bispoly]bis(2,2,2'trifluoroethylene) phosphazine and the like; metal polymers such as those derived by condensation of trans-bis(tri-n-butylphosphine)platinum dichloride with a bisacetylene or trans-bis(tri-n-butylphosphine)bis(1,4-butadinynyl) platinum and similar combinations in the presence of cuprous iodine and an amide; cellulose and cellulose derivatives such as esters of cellulose as for example triacetate cellulose, acetate cellulose, acetate-butyrate cellulose, nitrate cellulose, and sulfate cellulose, ethyl ether cellulose, hydroxypropyl ether cellulose, carboxymethyl ether cellulose, ethyl hydroxyethyl ether cellulose, cyanoethylethyl ether cellulose, acetoxyethyl ether cellulose , benzoyloxypropyl ether cellulose, phenyl urethane cellulose and the like; thermotropic copolyesters as for example copolymers of 6-hydroxy-2-naphthoic acid and p-hydroxy benzoic acid, copolymers of 6-hydroxy-2-naphthoic acid, terephthalic acid and p-amino phenol, copolymers and 6-hydroxy-2-naphthoic acid, terephthalic acid and hydroquinone, copolymers of 6-hydroxy-2-naphthoic acid, p-hydroxy benzoic acid, hydroquinone and terephthalic acid, copolymers of 2,6-naphthalene dicarboxylic acid, terephthalic acid, isophthalic acid and hydroquinone, copolymers of 2,6-napthalene dicarboxylic acid and terephthalic acid, copolymers of p-hydroxybenzoic acid, terephthalic acid and 4,4'-dihydroxydiphenyl, copolymers of p-hydroxybenzoic acid, terephthalic acid, isophthalic acid and 4,4'-dihydroxydiphenyl, p-hydroxybenzoic acid, isophthalic acid, hydroquinone and 4,4'-dihydroxybenzophenone, copolymers of phenylterephthalic acid and hydroquinone, copolymers of chlorohydroquinone, terephthalic acid and p-acetoxy cinnamic acid, copolymers of chlorophydroquinone, terephthalic acid and ethylene dioxy-4,4'-dibenzoic acid, copolymers of hydroquinone, methylhydroquinone, p-hydroxybenzoic acid and isophthalic acid, copolymers of (l-phenylethyl)hydroquinone, terephthalic acid and hydroquinone, and copolymers of poly(ethylene terephthalate) and p-hydroxybenzoic acid; and thermotropic polyamides and thermotropic copoly(amide-esters). In the most preferred embodiments of the invention, composite articles include a filament network, which may include a high molecular weight polyethylene fiber, nylon 6 or nylon 66 fiber, an aramid fiber, a fiber formed from liquid crystalline polymers such as liquid crystalline copolyester and mixtures thereof. US-A-4,457,985 generally discusses such high molecular weight polyethylene fibers and the disclosure of this patent is hereby incorporated by reference to the extent that it is not inconsistent herewith. In the case of polyethylene, suitable filaments are those of molecular weight of at least 150,000, preferably at least one million and more preferably between two million and five million. Such extended chain polyethylene (ECPE) filaments may be grown in solution as described in US-A-4,137,394 or 4,356,138 or a filament spun from a solution to form a gel structure, as described in DE-A- 3,004,699 and GB-A-2051667, and especially described in US-A-4,551,296 (see EP-A-64,167, published Nov. 10, 1982). As used herein, the term polyethylene shall mean a predominantly linear polyethylene material that may contain minor amounts of chain branching or comonomers not exceeding 5 modifying units per 100 main chain carbon atoms, and that may also contain admixed therewith not more than 50 wt% of one or more polymeric additives such as alkene-1-polymers, in particular low density polyethylene, polypropylene or polybutylene, copolymers containing mono-olefins as primary monomers, oxidized polyolefins, graft polyolefin copolymers and polyoxymethylenes, or low molecular weight additives such as anti-oxidants, lubricants, ultra-violet screening agents, colorants and the like which are commonly incorporated by reference. Depending upon the formation technique, the draw ratio and temperatures, and other conditions, a variety of properties can be imparted to these filaments. The tenacity of the filaments should be at least 15 grams/denier (13.2 dN/tex), preferably at least 20 grams/denier (17.7 dN/tex), more preferably at least 25 grams/denier (22.1 dN/tex) and most preferably at least 30 grams/denier (26.5 dN/tex). Similarly, the tensile modulus of the filaments, as measured by an Instron tensile testing machine, is at least 300 grams/denier (264.9 dN/tex), preferably at least 500 grams/denier (441 dN/tex) and more preferably at least 1,000 grams/denier (883 dN/tex) and most preferably at least 1,200 grams/denier (1059.6 dN/tex). These highest values for tensile modulus and tenacity are generally obtainable only by employing solution grown or gel filament processes. In the case of aramid fibers, suitable aramid fibers formed principally from aromatic polyamide are described in US-A-3,671,542. Preferred aramid fibers will have a tenacity of at least 20 g/d (17.7 dN/tex), a tensile modulus of at least 400 g/d (353.2 dN/tex) and an energy-to-break at least 8 joules/gram, and particularly preferred aramid fibers will have a tenacity of at least 20 g/d (17.7 dN/tex), a modulus of at least 480 g/d (423.8 dN/tex) and an energy-to-break of at least 20 joules/gram. Most preferred aramid fibers will have a tenacity of at least 20 g/denier (17.7 dN/tex), a modulus of at least 900 g/denier (794.7 dN/tex) and an energy-to-break of at least 30 joules/gram. For example, poly(phenylene terephthalamide) fibers produced commercially by Dupont Corporation under the trade name of Kevlar 29, 49, 129 and 149 having moderately high moduli and tenacity values are particularly useful in forming ballistic resistant composites. Also useful in the practice of this invention is poly(metaphenylene isophthalamide) fibers produced commercially by Dupont under the trade name Nomex. In the case of liquid crystal copolyesters, suitable fibers are disclosed, for example, in US-A-3,975,487; 4,118,372; and 4,161,470. Tenacities of 15 to 30 g/d (13.2 to 26.5 dN/tex) and preferably 20 to 25 g/d (17.7 to 22.1 dN/tex), and modulus of 500 to 1500 g/d (441.5 to 1324.5 dN/tex) and preferably 1000 to 1200 g/d (883 to 1059.6 dN/tex), are particularly desirable. Fibers in fibrous layers 12 may be arranged in networks (which can have various configurations) embedded or substantially embedded in a polymeric matrix which preferably substantially coats each filament contained in the fiber bundle. The manner in which the fibers are dispersed or embedded in the polymeric matrix may vary widely. For example, a plurality of filaments can be grouped together to form a twisted or untwisted yarn bundle in various alignments. The fibers may be formed as a felt, knitted or woven (plain, basket, satin and crow feet weaves, etc.) into a network, fabricated into non-woven fabric, arranged in parallel array, layered, or formed into a woven fabric by any of a variety of conventional techniques. Among these techniques, for ballistic resistance applications we prefer to use those variations commonly employed in the preparation or aramid fabrics for ballistic-resistant articles. For example, the techniques described in US-A- 4,181,768 and in M.R. Silyquist et al., J. Macromol Sci. Chem ., A7(1), pp. 203 et. seq. (1973) are particularly suitable. In preferred embodiments of the invention, the fibers in each layer 12 are aligned substantially parallel and undirectionally to form a uniaxial layer 12 such as in a prepreg, pultruded sheet and the like. Wetting and adhesion of fibers in the polymer matrices, is enhanced by prior treatment of the surface of the fibers. The method of surface treatment may be chemical, physical or a combination of chemical and physical actions. Examples of purely chemical treatments are use of SO 3 or chlorosulfonic acid. Examples of combined chemical and physical treatments are corona discharge treatment or plasma treatment using one of several commonly available machines. The matrix material may vary widely and may be formed of any thermoplastic polymer,thermosetting resin or a mixture thereof. Suitable polymeric matrix materials include those mentioned below for use in the formation of the fibers of layer 12. Useful matrix polymer materials may exhibit relatively high modulus e.g.equal to or less than 500 psi (3450 kPa) or may exhibit relatively high modulus e.g. greater than 500 psi ( 3450 k Pa ). In one preferred embodiment of the invention the matrix material is a relatively high modulus blend of one or more thermoplastic polymers and one or more thermosetting resins. The choice of thermoplastic polymer and thermosetting resin and their relative amounts may vary widely depending on the desired characteristics of the composite. Useful matrix materials are described in more detail in WO 91/08895 and are preferably a mixture of thermosetting vinyl ester resin and a thermoplastic polyurethane. In another preferred embodiment of this invention the matrix material is selected from the group consisting of relatively low modulus elastomeric materials. A wide variety of elastomeric materials and formulation may be utilized in the preferred embodiments of this invention. Representative examples of suitable elastomeric materials for use in the formation of the matrix are those which have their structures, properties, and formulation together with cross-linking procedures summarized in the Encyclopedia of Polymer Science, Volume 5 in the section Elastomers-Synthetic (John Wiley & Sons Inc., 1964)and those which are described in US-A-4,916,000 and are preferably block copolymers of conjugated dienes such as butadiene and isoprene are vinyl aromatic monomers such as 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, multiblock 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 the Shell Chemical Co. and described in the bulletin "Kraton Thermoplastic Rubber", SC-68-81. The volume ratios of resin to fiber may vary. In general, the volume percent of the resin may vary from 5 to 70 vol.% based on the total volume of layer 12. In the preferred embodiments of the invention, the volume percent of the resin is from 5 to 50 vol.%, in the more preferred embodiments of this invention is from 10 to 40 vol.% and in the most preferred embodiments of this invention is from 15 to 30 vol.% on the aforementioned basis. Layers 12 can be fabricated using conventional procedures. For example, layer 12 is formed by making the combination of fibers and matrix material in the desired configurations (such as a woven or non-woven fabric and layers in which fibers are aligned in a substantially parallel, unidirectional fashion), and amounts, and then subjecting the combination to heat and pressure using conventional procedures as for example those described in U S -A-4,916,000; 4,403,012; 4,737,401; 4,623,574; and 4,501,856; and WO 91/08895. Composite 10 can be formed by any conventional procedure. For example, one such procedure involves pre-forming a multilayer laminate and thereafter subjecting the laminate to a suitable stitching procedure such as sewing or drilling holes and guiding yarn through the holes to form composite 10 in which the stitch length is greater than the stitch path. One such procedure where composite 10 comprises fibrous layers where the fiber network is a knitted, woven or non-woven fabric involves aligning the desired number of layers formed of knitted, woven or non-woven fabric in a polymeric matrix and thereafter molding said aligned layers 12 at a suitable temperature and pressure to form a laminate structure of the desired thickness which can be stitched employing suitable stitching means such that the stitch length is greater than the stitch path. Another suitable procedure is where composite 10 comprises a laminate comprised of a plurality of layers 12 in which the polymer forming the matrix coats or substantially coats the filaments of multi-filament fibers and the coated fibers are arranged in a sheet-like array and aligned parallel to < another along a common fiber direction. Successive layers of such coated, uni-directional fibers can be rotated with respect to the previous layer to form a laminated structure 10. An example of such laminate structures are composites with the second, third, fourth and fifth layers rotated +45°, -45°, 90° and 0°, with respect to the first layer, but not necessarily in that order. Other examples include composites with 0°/90° layout of yarn or filaments. The laminates composed of the desired number of layers 12 can be molded at a suitable temperature pressure to form a precomposite having a desired thickness. Techniques for fabricating these laminated structures are described in greater detail in U.S. Patent Nos. 4,916,000; 4,623,574; 4,748,064; 4,457,985 and 4,403,012. The laminated layers can be stitched together using a suitable stitching means such that the stitch length is greater than the stitch path. Suitable stitching means include sewing machines, combination of drills and needles,and the like to form composite 10. In the preferred embodiment of this invention, composite 10 is prepared by the process of this invention. The first step of this process involves forming a laminate formed of a plurality of layers 12 by aligning adjacent layers in the desired configuration as described above and molding said aligned layers at a first temperature and first pressure sufficiently low to form a laminate having a thickness, t 1 , and which is such that the laminate can be stitched by a suitable stitching means such that the first stitch length of said laminate is equal to or substantially equal to the first stitch path of said laminate as depicted in FIG.4. First pressures and first temperatures employed in the formation of the laminate are preferably equal to or less than 80°C and 3,000, kPa, respectively, more preferably from 20°C to 50°C and from 200 to 2,000 kPa, respectively, and most preferably from 20°C to 30°C and 300 to 1,000 kPa, respectively. The laminate is stitched using the desired stitching means such that the stitch length is equal to or substantially equal to the stitch path and such that the laminate has a thickness, t 1 . The laminate is then molded at a second temperature which is greater than the first temperature and/or a second pressure which is greater than the first pressure to form the composite of this invention having a thickness, t 2 , which is less than the thickness, t 1 , of said laminate, and having a second stitch path which is less than the first stitch path of said laminate and having a second stitch length which is equal to or substantially equal to said first stitch length of said laminate and which is greater than the second stitch path of said laminate. Second temperatures and second pressures are preferably equal to or greater than 50°C and 700 kPa, respectively. Second temperatures and second presssures are more preferably from 80°C to 400°C and from 700kPa to 30,000 kPa, respectively, and most preferably from 110°C to 350°C and from 5,000 to 20,000 kPa, respectively. While we do not wish to be bound by any theory, it is believed that when the laminate is molded at the higher second temperature the viscosity of the polymeric matrix becomes lower at the higher second temperature such that under the molding pressure the matrix polymer flows to fill or substantially fill the voids and holes created during the stitching step resulting in enhanced penetration resistance. The molding procedure also compresses the laminate such that the thickness of the compressed composite is less than that of the laminate which also decreases the length of the stitch path of the resulting composite such that it is less than that of the laminate. The length of the fiber forming the stitch length in the laminate and in the resulting composite 10 are the same or substantially the same. In the resulting composite 10, that portion of the stitch length which exceeds the stitch path of the resulting composite 10 is retained or is substantially retained in the interior of composite 10. FIG 2 depicts an article 18 which differs from article 10 by the manner in which difference in the stitch path and stitch length is obtained. In composite 10 of FIG. 1 the difference is obtained by compressing the layers 12 of article 18 such that the length of the stitch length which exceeds the length of the path is inside of the compacted composite. However in article 10 , the length of the stitch length which exceeds the stitch path is external to the composite. The composites of this invention can be used for conventional purposes using conventional fabrication procedures. For example, such composites can be used in the fabrication of penetration and blast resistance articles and the like using conventional methods. The articles are particularly useful as vehicular armor or penetration resistant articles such as armor for tanks, airplanes, helicopters, armored personnel carriers and the like. The composite of this invention can be conveniently used for such purposes using conventional procedures and vehicles. The composite of this invention may also be used in the fabrication of blast resistant articles such as cargo compartments of aircraft, containers for explosives and the like. The following examples are presented to provide a more complete understanding of the invention and are not to be construed as limitations thereon. COMPARATIVE EXAMPLE I A ballistic composite was prepared from a plurality or stack of uniaxial prepreg sheets. Each uniaxial prepreg sheet comprised high strength extended chain polyethylene (ECPE) yarn, Spectra®-1000 (a product of Allied-Signal Inc.), impregnated with a Kraton D1107 thermoplastic elastomer (a polystyrenepolyisoprene-polystyrene block copolymer having 14 wt.% styrene and a product of Shell Chemical). Spectra®-1000 yarn has a tenacity of 33 grams/denier (gpd) (29.1 dN/tex), a modulus of 1,500 grams/denier (gpd) (1324.5 dN/tex) and energy to break of 49 joules/gram. The elongation to break of the yarn was 3%, denier was 1,300 (1450 tex) and an individual filament denier was 5.4 (0.6 tex), or 240 filaments per yarn end. Each filament has an approx. diameter of 0.001 in.(0.0026 cm). Thermoplastic elastomer resin impregnated uniaxial sheets were prepared as described in US-A-4 916 000. Resin coating system consisted of a resin applicator tube moving reciprocatingly across the width of the aligned Spectra®-1000 yarn while the liquid resin, Prinlin (TM) -B7137X-1, was pumped through the resin applicator tube. Prinlin is product of Pierce & Stevens Corp. which contains Kraton D-1107 in water emulsion, and the exact formulation is proprietary. Technical information about Kraton is described in the bulletin Kraton thermoplastic rubber, typical property guide Kraton D and Kraton G. It is indicated to have the NO. SC:68-81. Kraton D-1107 has a glass transition temp. of -55°C and a modulus tested using ASTM-D462 with a jaw separation speed of 10 in/ min. (25.4 cm/min) of 100 psi (690 Kpa) at 300% elongation. Prinlin resin was coated on the yarn web of approx. 24" (61 cm) wide X 0.0027" (0.0069 cm) thick and the yarn web consisted of aligned 228 ends of Spectra®-1000 yarn, or 9.5 yarn ends / inch-web/width (3.74 yarn ends/cm-web/width). The web coated with Prinlin was supported on a silicone release paper and was pulled, at a speed of 20 ft/min. (6.1 m/min), through a pair of nip rollers with a gap setting of approx. 0.026" (0.066 cm). The coated web passed through a gas fired oven at a temperature of 110°C where water vapor was vented out to ambient. The release paper was then peeled away from the resin impregnated sheet, or prepregs, measured 24" (61 cm) wide and thickness of 0.0026" (0.0066 cm). The prepreg contained 75% by weight of Spectra®-1000 and 25% by weight of Kraton D-1107 resin. The prepreg sheet was cut into 12" (30.5 cm) X 12" (30.5 cm) layers and a total of 364 layers were subsequently stacked into a preform. The fibers in adjacent layers were perpendicular to each other in 0°, 90°, 0°, 90°, etc. The preform was molded at 110°C under 50 tons sq. ft. (4,800 Kpa) pressure for about 30 minutes and cooled to room temperature. The molded panel, measured 12" (30.5 cm) X 12" (30.5 cm) X 1" (2.54 cm) thick, had an areal density of 5 pounds per square ft. (1 kg/m 2 ). The panel was impacted by a designated threat and the measured V/50 was 3,200 ft/sec (976 m/sec). Severe delamination and panel separation into numerous pieces were observed. EXAMPLE 1 Comparative Example 1 was repeated except that, prior to the molding of preform of 364 layers with 0°, 90°, 0°, 90°, .. orientation into a panel, the preform was lightly pressed under a pressure of 5 tons per square ft (480 kg/m 3 ). and room temperature of 23'C. The preform was measured approximately 12" (30.5_cm) X 12" (30.5 cm) X 1.25" (3.18 cm) thick and was stitched using Spectra®-1000 yarns of 7,800 denier (866.6 tex) with a breaking strength of approximately 570 lbs (260 kg). The stitched linear path was kept at 1" (2.54 cm) away from the peripheral edge of the preform. In addition to the peripheral stitch, the preform was also stitched vertically and horizontally to bisect the preform into four reinforced quadrants. The vertical stitched yarn penetrating through the preform was 1.25" (3.18 cm) while the yarn was loosely stitched horizontally along the top and bottom preform surfaces. The yarn lengths on top and bottom surface were 0.75" (1.9 cm) and 0.75" (1.9 cm), respectively, as compared to the shortest horizontal distance of 0.5" (1.27 cm) between the two needle stitching points. The cyclic yarn length was 4" (10.2 cm). The preform was molded, according to Comparative Example 1, into a panel of 12" (30.5 cm) X 12" (30.5 cm) X 1" (2.54 cm) thickness with an areal density of 5.2 lbs/ft 2 (1.04 kg/m 2 ). The cyclic stitched yarn length was 2.75" (7 cm) as compared to the shortest path of 3" (7.6 cm) = 1" (2.54 cm) x 2 (5.1 cm) (vertical) + 0.5" (1.3 cm) (top surface) +0.5" (1.3 cm) (bottom surface). The difference of the stitched yarn length (=4") (10.2 cm) and the shortest path length (=3") (7.6 cm) was 1" which allowed the panel, upon impact, to delaminate at a controlled level of 1" (2.54 cm) maximum. The panel was impacted against the same type of projectile as shown in Comparative Example 1, and the V/50 was 3,478 ft/sec (1,060 m/S). COMPARATIVE EXAMPLE 2 Comparative Example 1 was repeated except that the designated ballistic projectile impacted the panel without stitching at a velocity of 2,750 (838 m/sec). After the first shot, the projectiles only partially penetrate the panel. After three shots, the panel was completely delaminated into several pieces which was unsuitable for armor use. The maximum multiple hit capability was less than three. EXAMPLE 2 Example 1 was repeated except that the designated ballistic projectiles impacted the stitched panel at a velocity of 2,750 fps (838 m/sec). No panel penetration was observed under this velocity. After six shots no panel separation was observed due to loose stitching. The maximum multiple hit capability of the stitched panel was six (6) which shows a significant improvement over the panel without stitching described in Comparative Example 2.

Claims: A rigid penetration resistant composite having a plurality of fibrous layers comprising a network of fibers in a polymeric matrix selected from thermoplastic polymers, thermosetting resins or a combination thereof, , at least two of said layers being secured together by a stitching means which comprises fiber stitches characterized in that the average stitch length is greater than the average stitch path, the average stitch path being defined as the linear distance traversed by a stitch from a first exit of the stitch from a first surface of the composite, across said first surface to a first entrance through said first surface, from the first entrance through the composite to a second exit through the second surface, from said second exit across said second surface to a second entrance through said second surface, and through the composite to a third exit through said first surface, and the stitch length being the total length of fiber between said first and third exits. A composite according to Claim 1 characterized in that said stitching means comprises a plurality of first stitches extending along all or a portion of at least two adjacent or substantially adjacent paths. A composite according to Claim 1 characterized in that the tenacity of the fiber stitches is from 4 to 50 grams/denier (3.5 to 44.2 dN/tex) and said modulus is from 40 to 3000 grams/denier (35.3 to 2649.0 dN/tex). A composite according to Claim 1 characterized in that said fiber stitches are formed from fiber selected from the group consisting of polyethylene fiber, aramid fiber, nylon fiber, polyester fiber, glass fiber and combination thereof. A composite according to Claim 1 characterized in that the stitch length is from 102 to 300% of the stitch path. A composite according to Claim 1 characterized in that the stitch length is from 105 to 150% of the average stitch path. A composite according to Claim 1 characterized in that it further comprises parallel or substantially parallel second stitches, said first and second stitches intersecting at an angle. A composite according to Claim 1 characterized in that said angle is from 45° to 90°. A composite according to Claim 1 characterized in that said fibrous layers comprising fibers having a tensile strength of at least 7 grams/denier (6.2 dN/tex), a tensile modulus of at least 50 grams/denier (44.2 dN/tex) and an energy-to-break of at least 30 Joules/gram. A composite according to Claim 1 characterized in that said fibers are polyethylene fibers, nylon fibers, polyester fibers, glass fiber, aramid fibers or a combination thereof. A process for forming a composite according to Claim 2 which comprises the steps of: a) securing a plurality of adjacent fibrous layers each of which comprises a network of fibers dispersed in a polymeric matrix by a plurality of stitches having an average first stitch length and having an average stitch path wherein said first stitch length is equal to or substantially equal to said first path to form a composite having a first thickness, t 1 , and b) compressing said first composite at a temperature and pressure to form a penetration resistant second composite having a second thickness, t 2 , a second stitch length and a second stitch path wherein said first thickness, t 1 , is greater than said second thickness, said second stitch length is equal to or substantially equal to said first stitch length, and wherein said second stitch path is less than said first stitch path. An article of manufacture comprising a body which is formed totally or in part from a composite according to Claim 1.

Inventors: KWON YOUNG D, LEM KWOK W, LI HSIN L, PREVORSEK DUSAN C, KWON, YOUNG D., LEM, KWOK W., LI, HSIN L., PREVORSEK, DUSAN C., Kwon, Young D., c/o ALLIED-SIGNAL INC., Lem, Kwok W., c/o ALLIED-SIGNAL INC., Li, Hsin L., c/o ALLIED-SIGNAL INC., Prevorsek, Dusan C., c/o ALLIED-SIGNAL INC.
Applicants: ALLIED SIGNAL INC, ALLIEDSIGNAL INC.
Application year: 1993
Main classification: F41H5
IPC codes: F41H5