WO2007044041A2

WO2007044041A2 - Moisture-resistant pbo fiber and articles, and method of making

Info

Publication number: WO2007044041A2
Application number: PCT/US2005/045151
Authority: WO
Inventors: Brian D. Arvidson; Lori L. Wagner; Madhusudhan Rammoorthy; Harold Lindley Murray; Ashok Bhatnargar
Original assignee: Honeywell International Inc.
Priority date: 2004-12-29
Filing date: 2005-12-14
Publication date: 2007-04-19
Also published as: MX2007008028A; WO2007044041A3; EP1831635A2; IL184291A0; CN101133296A; JP2008525243A

Abstract

PBO fibers, PBO fiber-containing sheets and assemblies thereof, having improved of properties after exposure to high humidity.The PBO fibers are encapsulated by anhydrous means in a sealant material having a water vapor permeasbility of less than 25x10-11 cm3 (stp) cm/(cm2 sec.Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8°C, Inn the case of PBO fiber-containing sheets, the sealant material partially fills volume between the fibers.

Description

MOISTURE-RESISTANT PBO FIBER AND ARTICLES, AND METHOD OF MAKING

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to PBO fibers, fiber sheets comprising PBO fibers and to assemblies thereof, having improved retention of properties after exposure to high humidity. These fibers, sheets and articles find utility in applications requiring impact absorption, ballistic resistance and penetration resistance, as well as in other applications.

2. Description of the Related Art

The production of high strength fibers from materials such as aramids, high molecular weight polyethylene (HMWPE) and, more recently, polybenzazoles (PBO), has made possible and practical the fulfillment of a wide range of needs. Perhaps most dramatically, personal body armor constructed from these fibers has saved the lives of many of our police officers and military personnel.

Body armor is typically formed from layers of woven fabrics or non-woven sheets of fibers that are plied together. The fibers in a non- woven sheet may be unidirectionally oriented or felted in random orientation. Unidirectional fiber sheets (UD sheets) generally contain a matrix resin filling the volume between the fibers. Successive UD sheets are rotated relative to one another, for example at angles of 0°/90° or 0°/45^o/90^o/45^o/0° or at other angles. Composites made from cross-plied UD sheets generally have better ballistic resistance than woven fabrics, and therefore have a weight advantage. Rigid ballistic-resistant composites useful for applications such as riot shields and helicopter seats may also be constructed wherein individual sheets are bonded together using heat and pressure to adhere the matrix in each sheet, forming a bond between them, and consolidating the whole into a unitary article. With respect to current commercially available high strength fibers, PBO fibers have very high strength and tensile modulus, both of which are important in propagating and spreading the strain wave involved in a ballistic event. Unfortunately, however, PBO fibers may be weakened by exposure to high humidity (water in the vapor phase) with consequent deterioration of their anti-ballistic effectiveness. A publication titled, "PBO Fiber ZYLON®" Technical Information (Revised 2001.9) by Toyobo Co., LTD., shows that PBO fibers, after six weeks accelerated aging at 7O⁰C, 80% relative humidity, lose about 26% of their strength. It is often desirable that a body armor have sufficient "breathability" i.e., water vapor permeability, to afford an added level of comfort to the wearer. Construction of body armor from PBO fibers may thus require addressing two conflicting issues: protecting the PBO fibers from the effects of humidity while simultaneously providing water vapor permeability through the armor for the comfort of the wearer.

Ballistic-resistant and/or penetration-resistant articles comprising PBO woven fabrics are known in the prior art. See, e.g., USP's 6,559,079, 6,449,769, 6,238,768 and 5,552,221.

This prior art, alone or in combination, fails to describe the specific constructions of the articles or the methods of this invention and does not satisfy all of the needs met by this invention.

SUMMARY OF THE INVENTION

The present invention comprises a method of forming PBO fiber and PBO fiber-containing sheets, as well as assemblies thereof, having improved retention of properties after exposure to high humidity and temperature. These fibers, sheets and assemblies find utility in applications requiring impact absorption, ballistic resistance and penetration resistance, as well as in other applications.

In one embodiment, the invention is a method of forming a PBO fiber resistant to the effects of heat and humidity comprising the step of encapsulating the PBO fiber in a sealant material by anhydrous means. The sealant material has a water vapor permeability of less than 25 x 10^"11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C. This fiber can be formed into a variety of other articles.

In another embodiment, the invention is a method of forming a sheet, comprised of PBO fibers and resistant to the effects of heat and humidity, comprising the steps of: a) forming a fibrous sheet comprising a plurality of PBO fibers in a fiber network; b) encapsulating the PBO fibers in a sealant material by anhydrous means, the sealant material having a water vapor permeability of less than 25 x 10^~11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C, the sealant material partially filling the volume between the fibers, and in the case of a non-woven fiber network, additionally binding the fibers together; and c) optionally bonding a plastic film to at least one face of the fibrous sheet.

In yet another embodiment, the invention is a method of forming a ballistically resistant article retaining at least 87% of its initial V50 rating against a Geco 9mm, 124 grain, FMJ (steel jacket) bullet after accelerated aging for four weeks at 70⁰C, 80% relative humidity comprising the steps of: a) forming a fibrous sheet comprising PBO fiber in a fiber network; b) encapsulating the PBO fiber in a sealant material by anhydrous means, the sealant material having a water vapor permeability of less than 25 x 10^~11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C, the sealant material partially filling the volume between the fibers, and in the case of a non-woven fiber network, additionally binding the fibers together; c) optionally bonding a plastic film to at least one face of the fibrous sheet; and d) stacking a plurality of the fibrous sheets upon one another in unconnected or loosely connected array. In another embodiment, the invention is a method of forming a ballistically resistant article retaining at least 87% of its initial V50 rating against a Geco 9mm, 124 grain, FMJ (steel jacket) bullet after accelerated aging for four weeks at 7O⁰C, 80% relative humidity comprising the steps of: a) aligning a plurality of PBO fibers in a unidirectional planar sheet; b) encapsulating the PBO fibers in a sealant material by anhydrous means, the sealant material having a water vapor permeability of less than 25 x 10^"11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C, the sealant material partially filling the volume between the fibers, and binding the fibers together to form a coherent sheet; c) stacking a first and second of the coherent sheets upon one another, the direction of the fibers in the first sheet being at an angle of at least 30° to the direction of the fibers in the second sheet; d) bonding first and second coherent sheets together to form a laminate; e) optionally bonding a plastic film to at least one face of the laminate; and f) stacking a plurality of the laminates upon one another in unconnected or loosely connected array.

In other embodiments, the invention comprises the PBO fibers, fibrous sheets and articles comprised of one or more fibrous sheets comprised of PBO fibers in a fiber network. In each of these, the PBO fibers are encapsulated in a sealant material, as described above. The fibrous sheet can be formed from encapsulated PBO fibers or can be formed from PBO fibers and subsequently encapsulated. In the latter, subsequently- encapsulated fibrous sheet, the sealant material partially fills the volume between the fibersl, and in the case of non-woven sheets, the sealant material additionally binds the fibers together. The invention also includes assemblies of these articles.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a schematic representation of a process for making a unidirectional PBO fiber sheet of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, the invention is a method of forming a PBO fiber resistant to the effects of heat and humidity comprising the step of encapsulating the PBO fiber in a sealant material by anhydrous means. The sealant material has a water vapor permeability of less than 25 x 10^"11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C.

In another embodiment, the invention is a method of forming a sheet, comprised of PBO fibers and resistant to the effects of heat and humidity, comprising the steps of: a) forming a fibrous sheet comprising PBO fibers in a fiber network; b) encapsulating the PBO fibers in a sealant material by anhydrous means, the sealant material having a water vapor permeability of less than 25 x 10^"11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C, c) partially filling the volume between the fibers with the sealant material; and c) optionally bonding a plastic film to at least one face of the fibrous sheet.

In yet another embodiment, the invention is a method of forming a ballistically resistant article retaining at least 87% of its initial V50 rating against a Geco 9mm, 124 grain, FMJ (steel jacket) bullet after accelerated aging for four weeks at 70⁰C, 80% relative humidity comprising the steps of: a) forming a fibrous sheet comprising PBO fiber in a fiber network; b) encapsulating the PBO fiber in a sealant material by anhydrous means, the sealant material having a water vapor permeability of less than 25 x 10^"11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C, the sealant material partially filling the volume between the fibers, and in the case of a non-woven fiber network, additionally binding the fibers together; c) optionally bonding a plastic film to at least one face of the fibrous sheet; and d) stacking a plurality of the fibrous sheets upon one another in unconnected or loosely connected array. In another embodiment, the invention is a method of forming a ballistically resistant article retaining at least 87% of its initial V50 rating against a Geco 9mm, 124 grain, FMJ (steel jacket) bullet after accelerated aging for four weeks at 70⁰C, 80% relative humidity comprising the steps of: a) aligning a plurality of PBO fibers in a unidirectional planar sheet; b) encapsulating the PBO fibers in a sealant material by anhydrous means, the sealant material having a water vapor permeability of less than 25 x 10^~11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C, the sealant material partially filling the volume between the fibers, and binding the fibers together to form a coherent sheet; c) stacking a first and second of the coherent sheets upon one another, the direction of the fibers in the first sheet being at an angle of at least 30° to the direction of the fibers in the second sheet; d) bonding first and second coherent sheets together to form a laminate; e) optionally bonding a plastic film to one or both faces of the laminate; and f) stacking a plurality of the laminates upon one another in unconnected or loosely connected array.

In other embodiments, the invention comprises the fibers, fibrous sheets and articles comprised of one or more fibrous sheets comprised of PBO fibers in a fiber network. The fibers and/or the fibrous sheets are encapsulated in the previously described sealant material. In the case of fibrous sheets, the sealant material partially fills the volume between the fibers and, in the case of non-woven sheets, additionally binds the fibers together. The invention also includes assemblies of these articles. For purposes of the present invention, a fiber is an elongate body the length dimension of which is much greater than the transverse dimensions of width and thickness. Accordingly, "fiber" as used herein includes one, or a plurality of filaments, ribbons, strips, and the like having regular or irregular cross-sections in continuous or discontinuous lengths. A yarn is an assemblage of continuous or discontinuous fibers.

As used herein, "fiber network" or "network" denotes a plurality of fibers arranged into a predetermined configuration, or a plurality of fibers grouped together to form a twisted or untwisted yarn, which yarns are arranged into a predetermined configuration. The fiber network can have various configurations. For example, the fibers or yarn may be structured as a felt, knitted, braided, woven, randomly oriented non-woven, unidirectionally oriented non-woven, or formed into a network by any conventional techniques. According to a particularly preferred network configuration, the fibers are unidirectionally aligned so that they are substantially parallel to each other along the longitudinal direction of the network layer. The sheets and articles of the invention may additionally be comprised of other high strength fibers, such as high molecular weight polyethylene (HMPE), aramids and liquid crystal polyesters. When used- together with PBO fibers in unidirectional fiber orientation, as is preferred, these fibers are aligned parallel to the PBO fibers. It is also preferred that the fibers of different compositions are arranged in essentially periodic array in a direction transverse to the fiber direction. Hybrid fiber-containing sheet articles of the present invention are preferably comprised of from 10 to 100 percent of PBO fibers based on the total fiber weight. PBO fibers in the context of this invention are polybenzazole or polybenzoxazole fibers. PBO fibers suitable for the practice of this invention have been disclosed for example in USP's 5,185,296, 5,286,833, 5,356,584, 5,534,205, 5,976,447 and 6,040,050, hereby incorporated by reference. Preferably, the PBO fibers are ZYLON® brand poly(p- phenylene-2,6-benzobisoxazole) fibers commercially available from Toyobo Co., LTD.

HMWPE fibers useful in this invention have an intrinsic viscosity in decalin at 135°C of from about 5 deciliter/gram (dl/g) to about 35 dl/g. Such high molecular weight polyethylene fibers are commercially available under the SPECTRA® trademark from Honeywell International Inc. The disclosure of USP 4,413,110 is hereby incorporated by reference to the extent that it is not inconsistent herewith. The HMWPE fibers may also be produced by a rolling and drawing process as described in USP 5,702,657 and sold under the TENSYLON® brand by ITS Industries Inc. Aramid fibers useful in this invention are described in USP

3,671 ,542 and are commercially available from E.I. Dupont Co. under the trade names of KEVLAR® and NOMEX®; from Teijin Twaron BV under the trade names TWARON®, TECHNORA® and TEIJINCONEX®; from JSC Chim Volokno under the name ARMOS; and from Kamensk Volokno JSC under the names RUSAR and SVM. Poly(p-phenylene terephthalamide) and p-phenylene terephthalamide aramid co-polymer fibers having moderately high moduli and tenacity values are particularly useful in the present invention. An example of a p-phenylene terephthalamide copolymer aramid useful in the invention is co-poly- (paraphenylene 3,4'-oxydiphenylene terephthalamide). Also useful in the practice of this invention are poly(m-phenylene isophthalamide) fibers.

Liquid crystal copolyester fibers suitable for the practice of this invention are disclosed, for example, in USP's 3,975,487, 4,118,372 and 4, 161 ,470.

Anhydrous means for encapsulating the PBO fibers in the sealant material include, for example, chemical vapor deposition, vapor phase polymerization and deposition, in situ polymerization in an anhydrous solution followed by drying, coating by anhydrous sealant solution followed by drying, and other means not involving contact of the fibers with aqueous media such as solutions or dispersions. The disclosures of USP's 4,624,867, 5,447,799, and 6,179,922 describing means of vapor deposition of polymers on substrates, and USP Publication 2002/0002219 A1 describing means of in situ polymerization of fluoropolymers in porous substrates are hereby incorporated by reference to the extent not incompatible herewith. The preferred means of encapsulating the PBO fibers is coating of the PBO fibers with an anhydrous solution of the sealant material followed by drying.

The preferred method of the invention is schematically illustrated in Figure 1 for a unidirectional fiber network. It will be evident to one skilled in the art what modifications of the illustrated apparatus should be made to accommodate a woven, knitted, braided, randomly oriented planar (e.g. air-laid) or felted network.

PBO fibers are supplied from a creel 102 and passed through a combing station 104 to form a unidirectional network. The fiber network is then passed around stationary bars 20 to spread the yarns into thin layers. The fiber network is then carried under a roll immersed in a bath 105 of an anhydrous solution of the sealant material to completely coat each and every filament. The concentration of sealant material in the anhydrous solvent is such that when the solvent is dried, the sealant material does not completely fill the volume between the filaments. The concentration of the sealant solution is selected in relation to the water vapor permeability of the sealant. The lower the water vapor permeability of the sealant, the lower can be the sealant solution concentration. When working with a non-woven fiber network such as unidirectionally aligned fibers, the sealant concentration is also subject to the constraint that it must be sufficient to bind the fibers together in a coherent sheet for structural integrity. Preferably the concentration of the sealant material in the coating solution is from 1 % to 25% by weight of the solution (1 to 25 wt.%), more preferably from 5 to 20 wt.%. A squeeze roll 106 at the exit of the bath removes excess sealant solution from the fiber network.

The coated fiber network is mated with a carrier web 107 that can be a paper, a plastic film or a fabric. The coated fiber network is then passed through a heated oven 112 to evaporate the solvent in the sealant solution thereby encapsulating the fibers in the sealant material. The sealant material partially fills the volume between the fibers and binds the fibers together to form a coherent sheet. A nip roller 116 is used to pull the carrier web and fiber sheet through the system. The substrate and the fiber sheet are wound on a roller 118 in preparation for later construction of a laminate or a composite of the invention. The carrier web may be stripped from the fiber sheet or it may become a part of the final laminate or composite. The sealant material serves as a moisture barrier to protect the

PBO fibers and may also serve as a binder to provide coherence to a non- woven fiber sheet. The sealant material has a water vapor permeability of less than 25 x 10^'11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM Standard Test Method E 96-95 (Procedure E) at 37.8⁰C, more preferably less than 5 x 10^"11 cm³(stp) cm/(cm² sec-Pa). The abbreviation "stp" in the units of permeability has the commonly understood meaning of "standard temperature and pressure".

The type of sealant material is selected according to the end use requirements for articles constructed from sheets of the invention. If a flexible article is needed, the sealant material is preferably an elastomer with an initial tensile modulus less than 6,000 psi (41.4 MPa) as measured by ASTM D638. If a rigid article is needed, rigidity may be obtained either by plying sufficient numbers of sheets having a low modulus sealant material, or with fewer sheets having a high modulus resin sealant. When the article is a laminate used in a structural composite, it is preferred that the sealant material has an initial tensile modulus greater than 1 x 10⁶ psi (6.9 GPa) as measured by ASTM D638.

A variety of elastomeric materials and formulations having appropriately low water vapor permeability and modulus may be utilized as the sealant material in this invention. For example, the following materials are suitable: polyisoprene, natural rubber, ethylene-propylene copolymers, ethylene-propylene-diene terpolymers, polychloroprene (Neoprene G), poly(isobutylene-co-isoprene) (butyl rubber), styrene-isoprene-styrene block copolymer (KRATON® D1107 brand), styrene-(ethylene-co- butylene)-styrene block copolymer (KRATON® G1650 brand), and polytrichlorofluoroethylene. The water vapor permeabilities of some of these elastomers have been reported, for example, in "Polymer Handbook", 2^nd Ed., J. Brandrup and E. H. Immergut, Editors, Pages III-229 to III-250, John Wiley & Sons, New York 1975 and (1965), and "Fact Sheet K0102 Gas Permeability of KRATON Polymers", Kraton Polymers, Inc. and are shown in Table I. Table

High modulus sealant resins that may be useful in a laminate of the invention include thermoset allyls, aminos, cyanates, epoxies, phenolics, unsaturated polyesters, bismaleimides, rigid polyurethanes, silicones, vinyl esters and their copolymers and blends thereof. Thermoset vinyl ester resins are preferred.

Preferably, the vinyl ester is one produced by the esterification of a polyfunctional epoxy resin with an unsaturated monocarboxylic acid, usually methacrylic or acrylic acid. Illustrative vinyl esters include diglycidyl adipate, diglycidyl isophthalate, di-(2,3-epoxybutyl) adipate, di- (2,3-epoxybutyl) oxalate, di-(2,3-epoxyhexyl) succinate, di-(3,4-epoxybutyl) maleate, di-(2,3-epoxyoctyl) pimelate, di-(2,3-epoxybutyl) phthalate, di- (2,3-epoxyoctyl) tetrahydrophthalate, di-(4,5-epoxydodecyl) maleate, di- (23-epoxybutyl) terephthalate, di-(2,3-epoxypentyl) thiodiproprionate, di- (5,6-epoxytetradecyl) diphenyldicarboxylate, di-(3,4-epoxyheptyl) sulphonyldibutyrate, tri-(2,3-epoxybutyl)-1 ,2,4-butanetricarboxylate, di- (5,6-epoxypentadecyl) maleate, di-(2,3-epoxybutyl) azelate, di-3,4- epoxypentadecyl) citrate, di-(4,5-epoxyoctyl) cyclohexane-1 ,3- dicarboxylate, di-(4,5-epoxyoctadecyl) malonate, bisphenol-A-fumaric acid polyester and similar materials. Most preferred are epoxy based vinyl ester resins, such as the DERAKANE® resins manufactured by Dow Chemical Company.

The anhydrous solvent is chosen to be a good solvent for the sealant material. An anhydrous solvent in the context of this invention is one containing less than 0.5 wt.% water. Preferably, the anhydrous solvent contains less than 0.1 wt.% water. Most preferably, the anhydrous solvent contains less than 0.01 wt.% water. Solvents for several polymers useful as sealant materials are tabulated for example in "Polymer Handbook", 2^nd Ed., J. Brandrup and E. H. Immergut, Editors, Pages IV- 241 to IV-262, John Wiley & Sons, New York 1975, hereby incorporated herein by reference. Preferably, the anhydrous solvent has an atmospheric boiling point less than 15O⁰C, more preferably less than 100⁰C, so as to be readily evaporated. A preferred anhydrous solvent for a hydrocarbon elastomeric sealant is cyclohexane. A preferred anhydrous solvent for epoxy based vinyl ester resins is methyl ethyl ketone.

The invention is also a fibrous sheet comprised of PBO fibers in a fiber network, the fibers encapsulated in a sealant material having a water vapor permeability of less than 25 x 10^~11 cm³(stp) cm/(cm² sec Pa), as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C, and the sealant material fills between 5 and 99, preferably between 15 and 95, percent of the volume between the fibers. It is also preferred that the sealant material be deposited from an anhydrous solution followed by drying. tThe sealant material preferably comprises 1 to 50, more preferably 5 to 25, most preferably 5 to 20, percent by weight of the fiber sheet. Optionally, a plastic film is bonded to one or both faces of the fibrous sheet.

One article of the invention is a plurality of the fibrous sheets of the invention in stacked array. In a preferred embodiment, the fibers comprising the fibrous sheets are unidirectionally oriented parallel to one another, the fiber direction in a given sheet being at an angle to the fiber directions in adjacent sheets. The fibrous sheets may be unconnected, loosely connected to each other, or bonded together. A preferred article of the invention comprises unidirectional fibrous sheets that are cross-plied and bonded to one another. The cross-plying may be done by a continuous cross-plying operation such as described in USP's 5,173,138 or 5,766,725, hereby incorporated by reference to the extent not incompatible herewith, or by hand lay-up, or by any suitable means. In a preferred embodiment, the invention comprises a laminate comprised of first and second unidirectional fibrous sheets of the invention bonded together in stacked array, wherein the fiber direction in the first sheet is at an angle of at least 30° to the fiber direction in the second sheet. A preferred article of the invention comprises a plurality of these laminates in stacked array, unconnected or loosely connected to each other. In another embodiment, a laminate of the invention comprises one or more PBO fibrous sheets of the invention interleaved with one or more fibrous sheets of high strength fibers of other compositions bonded together in stacked array, optionally having a plastic film bonded to one or both faces of the laminate. A preferred form of this embodiment is a laminate comprising in sequence a first, second, third, and fourth sheet; said first and fourth lamina being comprised of aramid fibers; said second and third sheets being comprised of PBO fibers, and the sheets comprising the laminate being bonded to each other in stacked array. Most preferably, each of the fibrous sheets is comprised of unidirectional fibers with the fiber directions in adjacent sheets being normal to each other.

Optionally, a plastic film may be bonded to one or both faces of an article of the invention. Preferably, the plastic film is selected from a member selected from the group consisting of polyolefin, polyamide, polyester, polycarbonate, ionomer, cellulose, cellulose ester, ethyl cellulose and polyfluorocarbon. The plastic film, if present, preferably comprises from 1 to 40 percent by weight of the article. Preferably, the plastic film has a water vapor permeance greater than or equal to 5 x 10^"9 cm³ (stp)/(cm-sec-Pa), more preferably greater than or equal to 50 x 10^" ⁹cm³(stp)/(cm-sec-Pa), as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C. As permeance is equal to permeability divided by film thickness, thinner films should be used with materials of lower permeability and the reverse. For example, for a 0.914 density polyethylene film having a permeability of 0.675 x 10^"11 cm³(stp) cm/(cm²-sec Pa), a film thickness less than or equal to 0.0005 inch (0.0014 cm) thickness is preferred. Most preferably, the plastic film is a porous film.

The bonding of the unidirectional PBO fiber sheets of the invention to form laminates of the invention is preferably done by application of heat and pressure. Temperatures of from about 90° to about 16O⁰C and pressures of from about 100 psi to about 2500 psi (69 - 17,000 kPa) are employed depending upon the type of sealant and plastic film present.

In another embodiment, the invention is an article comprising a plurality of the above described laminates of the invention in stacked array, wherein the laminates are unconnected or loosely connected to each other, i.e., connected only at their edges or corners. It will be understood that the number of laminates comprising the article and their dimensions will be determined by the nature of the application.

Preferably the article is penetration resistant and/or ballistically- resistant. Preferably, a penetration-resistant article of the invention meets at least the requirements of NIJ Standard 0115.00 for Type 1 stab protection. Preferably, a ballistically-resistant article of the invention meets at least the requirements of NIJ Standard 0101.04 Revision A for Type Ha body armor. More preferably, a ballistically-resistant article of the invention has a specific energy absorption of at least 300 J-m²/Kg and a V50 rating of at least 1300 ft/sec (396 m/sec) when impacted by a 9 mm Geco, 124 grain FMJ (steel jacket) bullet. The V50 rating is that velocity at which a projectile has a 50% probability of penetrating the article. Yet more preferably, a ballistic-resistant article of the invention retains at least 87% of its initial V50 rating after accelerated aging for four weeks at 7O⁰C, 80% relative humidity. Most preferably, a ballistic-resistant article of the invention retains at least 85% of its initial V50 rating after accelerated aging for six weeks at 7O⁰C, 80% relative humidity.

In another embodiment, the invention is a composite comprising a plurality of the unidirectional fiber sheets of the invention bonded together in stacked array, wherein the fiber direction in a given sheet is at an angle to the fiber directions in adjacent sheets.

The following examples are presented to provide a more complete understanding of the invention. The specific techniques, conditions, materials, proportions and reported data set forth to illustrate the principles of the invention are exemplary and should not be construed as limiting the scope of the invention.

EXAMPLES Comparative Example 1 A PBO unidirectional fiber sheet was prepared using the apparatus shown schematically in Figure 1. Several rolls of 1005 denier PBO fiber (ZYLON® AS brand from Toyobo Co., LTD) were supplied from a creel 102 and were passed through a combing station 104 to form a unidirectional network. The fiber network was passed over and under stationary bars 20 to spread the yarns into thin layers. The fiber network was then carried under a roll immersed in a bath of an aqueous dispersion of an elastomeric styrene-isoprene-styrene block copolymer sealant to completely coat each filament. The aqueous dispersion, designated PRINLIN®7137AL supplied by Sovereign Specialty Chemicals, Buffalo, NY, is described by the manufacturer as a "resin modified water based dispersion of Kraton® D1107" containing 41 to 45 wt% solids.

The coated fiber network was passed through a squeeze roll 106 at the exit of the bath to remove excess sealant dispersion. The coated fiber network was placed on a 0.35 mil (0.00089 cm) polyethylene film carrier web 107 and passed through a heated oven to evaporate the water and form a coherent fiber sheet containing 20% sealant by weight of sealant plus fiber. The total areal density of the fibers, sealant and plastic film was 48.5 g/m². The fiber areal density was 36 g/m². The areal density of the sealant in the fiber sheet was 9 g/m². The carrier web and fiber sheet were then wound up on a roller 118 in preparation for construction of laminates.

Two rolls of the sheet material described above were placed on the cross-plying machine described in USP 5,173,138. The sheet materials were cross-plied 0°/90°, PBO to PBO, and consolidated at a temperature of 115⁰C and under a pressure of 500 psi (3.5 MPa) to create a laminate with two identical PBO fiber laminae and polyethylene films on both faces. The laminate was cut into multiple pieces having lateral dimensions of 40 cm x 40 cm. Twenty-two were stacked together and stitched together around their perimeters to form a number of articles for ballistic testing. One set of articles was held at normal atmospheric conditions. Other articles were subjected to accelerated aging at 7O⁰C, 80% relative humidity for 4 weeks and for 6 weeks. All articles were returned to normal atmospheric conditions for a minimum of three days prior to ballistic testing. The results of the ballistic testing with 9 mm Geco, 124 gr. FMJ (steel jacket) bullets are shown in Table Il below. Example 1

A PBO unidirectional fiber sheet was prepared as described in Comparative Example 1 except that instead of a water dispersion, the sealant bath was a 20 wt.% solution of styrene-isoprene-styrene block copolymer elastomer (KRATON® D1107 from Kraton Polymers, Inc.) in anhydrous cyclohexane containing less than 0.1 wt.% water. The sealant material had a water vapor permeability of 21.5 x 10^~11 cm³(stp) cm/(cm²- sec-Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C and an initial tensile modulus of 200 psi (1.4 MPa) as measured by ASTM D638. The coated fiber network was placed on a 0.35 mil (0.00089 cm) polyethylene film carrier web 107 and passed through a heated oven to evaporate the cyclohexane and form a coherent fiber sheet wherein each filament was encapsulated in sealant, the volume between the fibers being partially filled by sealant. The coherent PBO fiber sheet of the invention contained 20% sealant by weight of sealant plus fiber. The plastic film carrier and the fiber sheet of the invention were then wound up on a roller 118 in preparation for construction of laminates of the invention. The fraction of the volume between the fibers filled by the sealant was determined as follows: The total areal density of the fibers, sealant and plastic film was 48.5 g/m². The fiber areal density was 36 g/m². The areal density of the sealant in the fiber sheet was 9 g/m². PBO fibers have a density of 1.54 g/cm³. The volume actually occupied by the PBO fibers in one square meter of the coated fiber sheet was therefore equal to 36 g /1.54 g/cm³ = 23.4 cm³. The sealant, KRATON® D1107 has a density of 0.92 g/cm³. The volume of sealant in one square meter of the coated fiber sheet was therefore equal to 9 g /0.92 g/cm³ = 9.8 cm³. The total thickness of the fiber sheet plus plastic film was measured to be 0.0043 cm. The thickness of the coated fiber sheet (fibers plus sealant) was the total thickness minus the thickness of the plastic film equal to 0.0043 cm - 0.00089 cm = 0.00341 cm. The volume of one square meter of the coated fiber sheet was therefore 0.00341 cm x 10⁴ cm² = 34.1 cm³. The difference between the volume of the coated fiber sheet and the volume of the fibers was the volume between the fibers = 34.1 cm³ - 23.4 cm³ = 10.7 cm³. The sealant volume was 9.8 cm³. Therefore, the sealant occupied 9.8/10.7 x 100 = 92 % of the volume between the fibers in the coherent PBO fiber sheet of the invention. Two rolls of the inventive sheet material described above were placed on the cross-plying machine described in U.S. Patent 5,173,138. The sheet materials were cross-plied 0°/90°, PBO to PBO, and consolidated at a temperature of 115⁰C and under a pressure of 500 psi (3.5 MPa) to create a laminate with two identical PBO fiber laminae and polyethylene films on both faces. The polyethylene films had a water vapor permeance of 7.5 x 10^"9 cm³ (stp)/(cm-sec-Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C. The laminate was cut into multiple pieces having lateral dimensions of 40 cm x 40 cm. A number of articles of the invention, each consisting of twenty-two such laminates, were formed by stacking and stitching them together around their perimeters. One set of articles was tested to provide the initial ballistic properties. Othe/ articles were subjected to accelerated aging at 70⁰C, 80% relative humidity for 4 weeks and for 6 weeks. All articles were returned to normal atmospheric conditions for a minimum of three days prior to ballistic testing. The results of the ballistic testing with 9 mm Geco, 124 gr. FMJ (steel jacket) bullets are shown in Table Il below.

Table Il

It will be seen that the articles of the invention, where the sealant was applied from solution in an anhydrous solvent, compared to articles where the sealant was applied from an aqueous dispersion, showed significantly greater retention of ballistic properties after exposure to high humidity. The articles of the invention retained more than 87% of their initial V50 rating after four weeks at 7O⁰C, 80% R. H. and more than 85% after six weeks. "Breathability" of the articles was provided by incompletely filling the volume between the fibers and by provision of sufficiently permeable surface films. The articles of the invention exhibited a specific energy absorption against 9 mm Geco 124 gr. FMJ (steel jacket) bullets of greater than 300 J- m²/Kg. The articles of the invention met at least the requirements of NIJ Standard 0101.04 Revison A for Type HA body armor. Example 2 A composite of the invention is formed from twenty-two of the laminates described in Example 1 consisting of two cross-plied PBO fiber laminae and polyethylene films on both surfaces. The laminates are bonded together by molding under a temperature of 115⁰C and a pressure of 1000 psi (6.9 MPa). It is believed that the composite meets the requirements of NIJ Standard 0101.04 Revison A for Type MA body armor, the requirements of NIJ Standard 0115.00 for Type 1 stab protection, and retains more than 87% of its initial V50 rating after four weeks at 7O⁰C, 80% R. H. Example 3 A PBO unidirectional fiber sheet is prepared as described in

Example 1 except that the sealant bath is a 10 wt.% solution of poly(isobutylene) elastomer (VISTANEX® PIB MM 1-100 from ExxonMobil Chemical Co.) in anhydrous cyclohexane containing less than 0.1 wt.% water. The sealant material has a water vapor permeability of 0.28 x 10^"11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM Standard Test Method E 96-95 (Procedure E) at 37.8⁰C and an initial tensile modulus less than 6,000 psi (41.3 MPa) as measured by ASTM D638.

The coated fiber network is placed on a porous polytetrafluoroethylene film carrier web 107 having a water vapor 5 permeance of greater than 50 x 10^~9 cm³ (stp)/(cm-sec-Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C. The coated fiber network and film carrier are passed through a heated oven to evaporate the cyclohexane and to form a coherent fiber sheet wherein each filament is encapsulated in sealant. The coherent PBO fiber sheet of the invention contains 10% i o sealant by weight of sealant plus fiber. The sealant occupies 51 % of the volume between the fibers. The plastic film carrier web and fiber sheet of the invention are then wound up on a roller 118 in preparation for construction of laminates of the invention.

Two rolls of the inventive sheet material described above are

15 placed on the cross-plying machine described in USP 5,173,138. The sheet materials are cross-plied 0°/90°, PBO to PBO, and consolidated at a temperature of 115⁰C under a pressure of 500 psi (3.5 MPa) to create a laminate with two identical PBO fiber laminae and porous plastic films on both surfaces. 0 The laminate was cut into multiple pieces having lateral dimensions of 40 cm x 40 cm. A number of articles of the invention, each consisting of twenty-two such laminates, were formed by stacking and sewing together around their perimeters. The articles of the invention so formed are believed to meet at least the requirements of NIJ Standard 5 0101.04 Revison A for Type MA body armor and to retain at least 87% of their initial V50 rating after exposure to 7O⁰C, 80% relative humidity for 4 weeks. Example 4

A PBO unidirectional fiber sheet is prepared as described in o Example 1 except that the polyethylene film carrier is a porous film. Two rolls of the inventive sheet material are placed on the cross-plying machine described in USP 5,173,138. The sheet materials are cross-plied 0°/90°, PBO to PBO, and consolidated at a temperature of 115⁰C and under a pressure of 500 psi (3.5 MPa) to create a laminate with two identical PBO fiber laminae and porous polyethylene films on both faces. The polyethylene films have a water vapor permeance of greater than 75 x 10^'9 cm³ (stp)/(cm-sec-Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C.

Twenty-two laminates are bonded together by molding under a temperature of 115⁰C and a pressure of 1000 psi (6.9 MPa) to form a composite of the invention. It is believed that the composite meets the requirements of NIJ Standard 0101.04 Revison A for Type HA body armor, the requirements of NIJ Standard 0115.00 for Type 1 stab protection, and retains more than 87% of its initial V50 rating after four weeks at 7O⁰C, 80% R. H. Example 5

A PBO woven fabric of 8 mil (0.02 cm) thickness and having an areal density of 136 g/m² (Hexcel-Schwebel style 530 fabric consisting of ZYLON® AS 500 denier fiber from Toyobo Co., LTD.) is passed under a roll immersed in a bath containing an 8 wt.% solution of styrene-isoprene- styrene block copolymer elastomer (KRATON® D1107 from Kraton

Polymers, Inc.) in anhydrous cyclohexane containing less than 0.1 wt.% water. Each filament of the fabric is completely coated with the solution. The fabric is passed through a squeeze roll to remove the excess solution, placed on a 0.35 mil (0.00089 cm) polyethylene film carrier web and passed through a heated oven to evaporate the solvent. The carrier web and the coated fabric of the invention are then wound up on a roll in preparation for construction of ballistic resistant articles as described here below and in the following Example 6.

The total areal density of the fibers and sealant is 143 g/m². The sealant areal density in the dried fabric is 7 g/m². The fraction of the volume between the fibers filled by the sealant is determined as follows: The volume occupied by one square meter of uncoated fabric is 100 cm x 100 cm x 0.02 cm = 200 cm³. The volume actually occupied by the fibers in one square meter of uncoated fabric is 136 g/1.54 g/cm³ = 88.3 cm³. The volume between the fibers is therefore 200 cm³ - 88.3 cm³⁼ 111. 7 cm³. The volume occupied by the sealant is 7g/0.92 g/cm³ = 7.6 cm³. Therefore, the sealant occupies 7.61 cm³/111.7 cm³ x 100 = 6.8% of the volume between the fibers.

Articles for ballistic testing are formed from twenty-two pieces of the coated fabric with the polyethylene film on one surface having lateral dimensions of 40 cm x 40 cm stacked together and stitched together around their perimeters. One such article is held at normal atmospheric conditions. Other such articles are subjected to accelerated aging at 7O⁰C, 80% relative humidity for 4 weeks and for 6 weeks. All articles are returned to normal atmospheric conditions for a minimum of three days prior to ballistic testing. It is believed that the articles of the invention meet the requirements of NIJ Standard 0101.04 Revison A for Type HA body armor, the requirements of NIJ Standard 0115.00 for Type 1 stab protection, and retains more than 87% of their initial V50 rating after four weeks at 7O⁰C, 80% R. H. Example 6

The polyethylene film carrier web is stripped from the PBO fabric of the invention prepared in Example 5 above. Eleven pieces having dimensions of 40 cm x 40 cm are cut from this fabric and each piece is sandwiched face-to-face between two pieces of aramid fabric (Hexel-Schwebel style 705 consisting of 850 denier KEVLAR® KM-2 fibers in a 31 x 31 per inch plain weave) having the same dimensions. Each of the eleven fabric sandwiches thus formed are compression molded at a temperature of 115⁰C and under a pressure of 500 psi (3.5 MPa) to create laminates of the invention. The eleven laminates are stacked together and stitched together around their perimeters to form another article of the invention. It is believed that this article of the invention meets the requirements of NIJ Standard 0101.04 Revison A for Type NA body armor, the requirements of NIJ Standard 0115.00 for Type 1 stab protection, and retains more than 87% of its initial V50 rating after four weeks at 7O⁰C, 80% R. H. Example 7

PBO fiber (ZYLON® AS) chopped into 51 mm lengths is obtained from Toyobo Co. LTD. The chopped fiber is formed into a randomly oriented non-woven sheet (batt) by an air-laying process. The batt is passed through a needling loom to mechanically orient fibers in the vertical direction and to entangle the fiber mass into a coherent felt fabric. The PBO felt fabric is passed under a roll immersed in a bath containing a 10 wt. % solution of poly(isobutylene) elastomer (VISTANEX® PIB MM I- 100 from ExxonMobil Chemical Co.) in anhydrous cyclohexane containing less than 0.1 wt.% water. The sealant material has a water vapor permeability of 0.28 x 10^~11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM Standard Test Method E 96-95 (Procedure E) at 37.8⁰C and an initial tensile modulus of less than 6,000 psi (41.3 MPa) as measured by ASTM D638. Each filament of the felt fabric is completely coated with the solution. The fabric is passed through a squeeze roll to remove the excess solution, placed on a 0.35 mil (0.00089 cm) polyethylene film carrier web and passed through a heated oven to evaporate the solvent. The sealant occupies 10% of the volume between the fibers. A porous 1 mil (0.0025 cm) polyethylene film is applied to the top surface of the felt fabric and the assembly is passed between heated rolls at a temperature of 115°C and under a pressure of 500 psi (3.5 MPa) to create a fibrous sheet of the invention.

Articles for ballistic testing are formed from twenty-two pieces of the coated felt fabric with polyethylene films on both surfaces having lateral dimensions of 40 cm x 40 cm stacked together and stitched together around their perimeters. One such article is held at normal atmospheric conditions. Other such articles are subjected to accelerated aging at 7O⁰C, 80% relative humidity for 4 weeks and for 6 weeks. All articles are returned to normal atmospheric conditions for a minimum of three days prior to ballistic testing. It is believed that the articles of the invention meet the requirements of NIJ Standard 0101.04 Revison A for Type MA body armor, the requirements of NIJ Standard 0115.00 for Type 1 stab protection, and retains more than 87% of their initial V50 rating after four weeks at 7O⁰C, 80% R. H. Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to but that further changes and modifications may suggest themselves to one skilled in the art, all falling with the scope of the invention as defined by the subjoined claims.

Claims

What is claimed is:

1. A method of forming a sheet, comprised of PBO fibers and resistant to the effects of heat and humidity, comprising the steps of: a) forming a fibrous sheet comprising a pluraltiy of PBO fibers in a fiber network; b) encapsulating the PBO fibers in a sealant material by anhydrous means, the sealant material having a water vapor permeability of less than 25 x 10^~11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C, c) partially filling the volume between the fibers with the sealant material,; and d) optionally bonding a plastic film to at least one face of the fibrous sheet.

2. The method of claim 1 , wherein the PBO fibers comprise from 10 to 100 percent by weight of the fiber content of the sheet.

3. The method of claim 1 , wherein the fiber network is selected from the group consisting of felted, knitted, braided, woven, randomly oriented planar non-woven and unidirectionally oriented non-woven.

4. The method of claim 1 , wherein the fibers in the network are unidirectionally aligned so that they are substantially parallel to each other.

5. The method of claim 1 , wherein said sealant material fills from 5% to 99% of the volume between the fibers.

6. The method of claim 1 , wherein said sealant material fills from 15% to 95% of the volume between the fibers.

7. The method of claim 1 , wherein said sealant material has a water vapor permeability of less than 5 x 10^"11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C.

8. The method of claim 1 , wherein the anhydrous means of encapsulating the fibers is selected from the group consisting of chemical vapor deposition, vapor phase polymerization, polymerization in an anhydrous solution followed by drying, and coating by an anhydrous sealant solution followed by drying.

9. The method of claim 1 , wherein the anhydrous means of encapsulating the fibers is coating by an anhydrous sealant solution followed by drying.

10. The method of claim 9, wherein the solvent in said anhydrous sealant solution has an atmospheric boiling point less than 150⁰C.

11. The method of claim 9, wherein the solvent in said anhydrous sealant solution has an atmospheric boiling point less than 100⁰C.

12. A method of forming a ballistically resistant article comprising PBO fibers and that retains at least 87% of its initial V50 rating after conditioning for four weeks at 70⁰C, 80% relative humidity, comprising the steps of: a) forming a fibrous sheet comprising a plurality of PBO fibers in a fiber network; b) encapsulating the PBO fibers in a sealant material by anhydrous means, the sealant material having a water vapor permeability of less than 25 x 10^"11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C, c) partially filling the volume between the fibers with the sealant material; d) optionally bonding a plastic film to at least one face of the fibrous sheet; and e) stacking a plurality of the fibrous sheets upon one another in unconnected or loosely connected array.

13. A method of forming a ballistically resistant article comprising PBO fibers and that retains at least 87% of its initial V50 rating after conditioning for four weeks at 70⁰C, 80% relative humidity, comprising the steps of: a) aligning a plurality of PBO fibers in a unidirectional planar sheet; b) encapsulating the PBO fibers in a sealant material by anhydrous means, the sealant material having a water vapor permeability of less than 25 x 10^~11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96- 95 (Procedure E) at 37.8⁰C andpartially filling the volume between the fibers; c) binding the fibers together to form a coherent sheet; d) stacking a first and second of the coherent sheets upon one another, the direction of the fibers in the first sheet being at an angle of at least 30° to the direction of the fibers in the second sheet; e) bonding first and second coherent sheets together to form a laminate; f) optionally bonding a plastic film to one or both faces of the laminate; and g) stacking a plurality of the laminates upon one another in unconnected or loosely connected array.

14. A fibrous sheet comprised of PBO fibers in a fiber network, said fibers being encapsulated in a sealant material having a water vapor permeability of less than 25 x 10^"11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C, wherein said sealant material fills between 5 and 99 percent of the volume between the fibers, and optionally having a plastic film bonded to one or both faces of the sheet.

15. The fibrous sheet of claim 20, wherein the fiber network is selected from the group consisting of felted, knitted, braided, woven, randomly oriented planar non-woven and unidirectionally oriented non-woven.

16. The fibrous sheet of claim 20, wherein the fibers in the network are unidirectionally aligned so that they are substantially parallel to each.

17. The fibrous sheet of claim 20, wherein said sealant material fills from 15% to 95% of the volume between the fibers.

18. The fibrous sheet of claim 20, wherein the PBO fibers are poly(2- phenylene-2,6-benzobisoxazole).

19. The fibrous sheet of claim 20, wherein the PBO fibers comprise from 10 to 100 % by weight of the fiber content of said article.

20. The fibrous sheet of claim 20, wherein said sealant material comprises from 1 to 50 percent by weight of the article.

21.The fibrous sheet of claim 20, wherein said sealant material comprises from 5 to 25 percent by weight of the article.

22. The fibrous sheet of claim 20, wherein said sealant material comprises from 5 to 20 percent by weight of the article.

23. The fibrous sheet of claim 20, wherein said sealant material has a water vapor permeability of less than 5 x 10^'11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C.

24. The fibrous sheet of claim 20, wherein said sealant material is an elastomeric material having an initial tensile modulus less than 6,000 psi (41.3 MPa) as measured by ASTM D638.

25. The fibrous sheet of claim 20, wherein the plastic film is a member selected from the group consisting of polyolefin, polyamide, polyester, polycarbonate, ionomer, cellulose, cellulose ester, ethyl cellulose and polyfluorcarbon.

26. The fibrous sheet of claim 20, wherein the plastic film has a water vapor permeance greater than or equal to 5 x 10^~9 cm³ (stp)/(cm-sec- Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C.

27. The fibrous sheet of claim 20, wherein the plastic film has a water vapor permeance greater than or equal to 50 x 10^"9 cm³ (stp)/(cm-sec- Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8⁰C.

28. The fibrous sheet of claim 20, wherein the plastic film is porous.

29. The fibrous sheet of claim 20, wherein the plastic film comprises from 1 to 40 percent by weight of the sheet.

30. An article comprising a plurality of the fibrous sheets of claim 20 in stacked array, wherein the fibrous sheets are unconnected or are loosely connected to each other.

31. The article of claim 36 meeting at least the requirements of NIJ Standard 0101.04 Revision A for Type Ha body armor.

32. The article of claim 36 having a specific energy absorption of at least

300 J-m²/Kg and a V50 rating of at least 1300 ft/sec (396 m/sec) when impacted by a 9 mm Geco, 124 grain, FMJ (steel jacket) bullet.

33. A laminate comprising one or more fibrous sheets of claim 20 interleaved with one or more fibrous sheets of high strength fibers of other compositions bonded together in stacked array, and optionally having a plastic film bonded to one or both faces of the laminate.

34. The laminate of claim 39 comprising in sequence a first, second, third, and fourth lamina; wherein said first and fourth lamina are comprised of aramid fibers and said second and third lamina are comprised of the fibrous sheets of claim 20.

35. The laminate of claim 40, wherein said first and fourth lamina are comprised of unidirectional aramid fibers, the fiber directions in said first and fourth lamina being normal to each other; said second and third lamina are comprised of the fibrous sheets of claim 22, the fiber directions in said second and third lamina being normal to each other; the fiber direction in said first lamina being normal to the fiber direction in said second lamina, the fiber direction in said third lamina being normal to the fiber direction in said fourth lamina.

36. A laminate comprising a plurality of the unidirectional fibrous sheets of claim 22 bonded together in stacked array, wherein the fiber direction in a given sheet is at an angle to the fiber directions in adjacent sheets, optionally having a plastic film bonded to one or both faces of the laminate.

37. The laminate of claim 42, wherein the sealant material has an initial tensile modulus equal to or greater than 1 x 10⁶ psi (6.9 GPa) as measured by ASTM D638.

38. The laminate of claim 42, wherein a first and second unidirectional fibrous sheet of claim 22 are bonded together in stacked array, the fiber direction in said first sheet being at an angle of at least 30° to the fiber direction in said second sheet.

39. An article comprising a plurality of the laminates of claim 44 in stacked array, wherein the laminates are unconnected or are loosely connected to each other.

40. The article of claim 42 meeting at least the requirements of NIJ Standard 0101.04 Revision A for Type Ma body armor.

41. The article of claim 42 having a specific energy absorption of at least 300 J-m²/Kg and a V50 rating of at least 1300 ft/sec (396 m/sec) when impacted by a 9 mm Geco, 124 grain, FMJ (steel jacket) bullet.

42. The article of claim 42 having 87% or greater retention of its initial V50 rating after accelerated aging for four weeks at 70°C, 80% relative humidity.

43. The article of claim 42 having 85% or greater retention of its initial V50 rating after accelerated aging for six weeks at 70⁰C, 80%.

44. The article of claim 42 meeting at least the requirements of NIJ Standard 0115.00 for Type 1 stab protection.

45. The fibrous sheet of claim 20 wherein the fibers have been encapsulated by an anhydrous process selected from the group consisting of chemical vapor deposition, vapor phase polymerization, polymerization in an anhydrous solution followed by drying, and coating by an anhydrous sealant solution followed by drying.

46. The fibrous sheet of claim 20 wherein the fibers have been encapsulated by coating with an anhydrous sealant solution followed by drying.

47. PBO fiber encapsulated in a sealant material having a water vapor permeability of less than 25 x 10^"11 cm³(stp) cm/(cm² sec Pa) as measured by ASTM E 96-95 (Procedure E) at 37.8°C.