Ballistic conversion efficiency of high performance fibers

Abstract

To provide a fibrous article with improved fiber tensile properties.SOLUTION: There is provided a fibrous article comprising one or more unidirectional fiber plies, each unidirectional fiber ply comprising a plurality of unidirectionally oriented fibers coated with a polymeric binder, wherein the fibrous article has been primed by being pressed at a pressure of less than 0.0345 MPa (5 psi), wherein the fibers have been axially extended in a compression apparatus, wherein the compression apparatus is a compression conveyor comprising two pressure exerting elements, wherein the fibers have an initial elongation-to-break value prior to being axially extended and a reduced elongation-to-break value after being axially extended, wherein the initial elongation-to-break value is greater than the reduced elongation-to-break value, wherein the reduced elongation-to-break value is from about 2.0% to about 3.9%, and wherein elongation-to-break values are measured in accordance with ASTM D2256.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present technology relates to improvements in ballistic composite materials, and in particular to an efficient method for improving the tensile properties of fibrous composite materials. [Background technology]

[0002] High performance thermoplastic fibers, such as polyolefin fibers, are widely used in the manufacture of composite structures. Various methods for producing high strength thermoplastic fibers are known, including, for example, U.S. Patent Nos. 4,413,110, 4,536,536, 4,551,296, 4,663,101, 5,032,338, 5,578,374, 5,736,244, 5,741,451, 5,958,582, 5,972,498, and 6,444. Nos. 8,359, 6,746,975, 6,969,553, 7,078,099, 7,344,668, 7,846,363, 8,361,366, 8,444,898, 8,747,715, and 9,365,953, the disclosures of which are incorporated herein by reference to the extent consistent herewith.

[0003] Each of these patents describes a gel spinning technique for producing high-strength filaments, in which the filaments are drawn, i.e., stretched, over a series of drawing rolls to improve their tensile strength properties. For example, U.S. Patent Nos. 4,413,110, 4,663,101, and 5,736,244 describe the stretching of a formed polyethylene gel precursor and a low-porosity xerogel obtained from the polyethylene gel precursor to form a high-tenacity, high-modulus fiber. U.S. Patent Nos. 5,578,374 and 5,741,451 describe the post-stretching of polyethylene fibers that have already been oriented by drawing at a specific temperature and draw rate. U.S. Patent No. 6,746,975 describes a high-tenacity, high-modulus multifilament yarn formed from a polyethylene solution via extrusion through a multi-orifice spinneret to form a fluid product. The fluid product is gelled, stretched, and formed into a xerogel. The xerogel is then subjected to a two-stage stretching process to form the desired multifilament yarn. U.S. Patent No. 7,078,099 describes drawn gel-spun multifilament yarns with increased molecular structure extension. The yarns are produced by an improved manufacturing process and drawn under special conditions to obtain multifilament yarns with high molecular weight and crystalline order. U.S. Patent No. 7,344,668 describes a process for drawing essentially diluent-free gel-spun polyethylene multifilament yarns in a forced convection air oven and the drawn yarns produced thereby. The process conditions of draw ratio, elongation, residence time, oven length, and feed rate are selected in specific relationships to each other to achieve improved efficiency and productivity. U.S. Patent Nos. 7,846,363, 8,361,366, 8,444,898, 8,747,715, and 9,365,953 each teach improvements in methods known in the art for improving fiber tensile properties and thereby improving the ballistic properties of armor composites formed from the improved fiber tensile properties.

[0004] A common processing step required by all of the above-referenced methods is fiber drawing, which is conventionally known to improve fiber strength and is important in the production of highly effective composite armor. However, in the process of drawing filaments using any such drawing method, the filaments may break as the fibers approach their elongation limits. Broken filaments tend to wind around one of the draw rolls, and as they wind around the draw roll, they often interfere with adjacent unbroken filaments on the same draw roll, breaking the adjacent filaments. The fibers also break. Ultimately, this can lead to the entire yarn breaking. When a yarn breaks, other yarns being simultaneously drawn by the same drawing device can also break, resulting in process failure and substantial product waste. In view of these issues, maximizing fiber drawing to maximize their tenacity can be inefficient and result in relatively low product yields. Therefore, it would be beneficial in the art if there were an alternative means to improve fiber tensile properties without the need for significant fiber drawing after initial production to avoid such fiber breakage and waste.

[0005] Maximizing fiber tensile properties, as well as strength in the presence of a matrix (binder), is important for the production of effective composite armor, especially in the production of effective armor from nonwoven unidirectional fibrous composites, where a binder is used to hold the fibers of the unidirectional tape together. However, composite strength does not strictly follow the rule of mixture; that is, composite strength is not simply a direct measure of the strength of the fiber and matrix components. In particular, during the processing of the individual components into a fabric, some loss of composite strength occurs relative to the strength of the raw material components, referred to in the art as conversion efficiency. This can occur for a variety of reasons, such as material degradation due to processing conditions or imperfect processing results during the fiber-to-composite conversion (such as fiber misalignment, void formation, or uneven application of coatings). This presents two significant problems. First, the extent of strength loss is unpredictable, making composite strength unpredictable. Second, to compensate for this inefficient conversion of strength from fiber to composite, more layers of material are required to maintain a desired level of ballistic performance when converting the composite into an armor article, which undesirably and unnecessarily increases the weight, bulk, and cost of the armor. A solution is needed to address this problem.

[0006] The present technology has been unexpectedly found to solve both of the manufacturing inefficiencies discussed above, namely, low fiber yield due to lost manufactured product, and low efficiency of strength conversion from fiber to composite, by limiting fiber elongation and maximizing fiber strength through alternative processing. Summary of the Invention

[0007] It has been found that the efficiency of conversion of fiber and binder strength to composite strength is improved, inter alia, by first priming (i.e., lightly pressing under mild conditions such as wet lamination) a single-ply unidirectional prepreg web, multiple adjacent / stacked unidirectional prepreg webs, prepreg unidirectional tape (single individual prepreg ply), or multiple adjacent / stacked prepreg plies, and then passing the primed web, individual plies, or multiple plies / webs through an isopressure press (double belt press). When multiple plies and / or webs are primed, the priming step bonds the plies / webs together, typically forming a prepreg composite web / fabric comprising two, four, six, or eight or more plies (layers) (also referred to herein as a "multi-ply fabric" or "multi-ply prepreg fabric") or a prepreg composite web / fabric comprising two, four, six, or eight total unidirectional webs (also referred to herein as a "multi-ply web" or "multi-ply prepreg web"). Each individual web is a single web layer / web ply. A multi-ply prepreg fabric or multi-ply prepreg web can be processed through an isopressure press by itself, with one or more other multi-ply prepreg fabrics or unidirectional prepreg webs, or with a single prepreg unidirectional tape (single individual ply) not previously combined with or attached to another web / individual ply.

[0008] In each embodiment, the prepreg web(s) / unidirectional single individual prepreg webs A ply / multi-ply prepreg fabric includes fibers coated with a polymeric binder, or includes fibrous tapes (containing fibers) or non-fibrous tapes (containing fibers) optionally coated with a polymeric binder. The binder coating is performed before subjecting the web(s) / individual plies / fabric to axial stretching in a compaction device (e.g., a compaction conveyor). By priming the web(s) / individual plies / plies, the unidirectional alignment of the component fibers / tapes is made more uniform so that all component fibers / tapes are straighter and more uniformly aligned in the same axial direction, improving the uniformity of the binder coating on the fibers or tapes (if present). In this regard, the fibers / tapes may move slightly when coated with resin. Priming the layers helps to more ideally align such fibers / tapes. This maximizes fiber / tape coverage in the unidirectional fiber plies and also adjusts areas of excess binder concentration, thereby maximizing protrusion engagement with the fibers / tapes, while straightening the fibers / tapes in preparation for axial stretching. After priming, the primed unidirectional web(s) / individual plies / multi-ply fabric are then subjected to axial extensional stress(es) in the axial direction of each web / individual ply. By axially stretching the fibers / tapes in the compaction device rather than completing the fiber elongation through conventional drawing, the yield loss problem associated with maximum fiber elongation is avoided. Additionally, axial stretching of the fibers / tapes in the compaction device under the conditions described herein avoids fiber / tape degradation and achieves improved conversion of the tensile properties of the fibers / tapes to the tensile properties of the final composite compared to conventional manufacturing methods, such as consolidation under pressures of 5 psi or more in a flatbed laminator, autoclave, or molding device.

[0009] More specifically, provided is a fibrous article comprising one or more unidirectional fiber plies, each unidirectional fiber ply comprising a plurality of unidirectionally oriented fibers coated with a polymeric binder, wherein the composite fibrous article is primed and the composite fibers are axially stretched in a compression device, the composite fibers having an initial elongation to break value before being axially stretched and a reduced elongation to break value after being axially stretched, the initial composite elongation to break value being greater than the reduced composite elongation to break value, and the reduced composite elongation to break value being between about 2.0% and about 3.9%.

[0010] Also provided is a method for improving fiber tensile properties of a fibrous prepreg composite, the composite method comprising: a) providing at least one fibrous prepreg composite comprising one or more unidirectional fiber plies, each of the composite plies comprising a plurality of unidirectionally oriented fibers coated with a polymeric binder, the composite fibers having a toughness of 7 g / denier or greater, a tensile modulus of 150 g / denier or greater, and an elongation to break of at least 2.0%, and the composite fibers having a softening temperature and a melting temperature; b) heating the fibers of the composite to a temperature above the softening temperature of the composite fibers but below the melting temperature of the composite fibers; c) subjecting the composite to an axial extensional stress while the composite fibers are at a composite temperature greater than the softening temperature of the composite fibers but less than the melting temperature of the composite fibers, wherein the fibers in each fiber ply are axially elongated by the composite axial extensional stress, thereby increasing the toughness of each fiber in each ply by at least 1% and decreasing the elongation to break of each fiber by at least 1%.

[0011] Also provided are composites and ballistic resistant articles formed according to the composite method. [Brief explanation of the drawings]

[0012] [Figure 1]FIG. 1 is a top perspective view of the formation of a unidirectional fiber ply. [Figure 2] FIG. 1 is a schematic perspective view of a unidirectional fiber array being coated with a binder resin by passing it through a dip coater, followed by bonding the coated array with a release film. [Figure 3] 1 is a schematic cross-sectional view of an isostatic belt press and composite material formed therein. [Figure 4] FIG. 1 is a schematic perspective view of the manufacture of a two-ply 0° / 90° web. DETAILED DESCRIPTION OF THE INVENTION

[0013] A method is provided for manufacturing high performance composite articles from one or more unidirectional fiber plies, where each "unidirectional fiber ply" may be in the form of a continuous web of unidirectionally oriented (aligned) fibers, or in the form of a single, individual unidirectional ply / unidirectional tape having a length, as described below. Prior to pressing in a compaction device, the unidirectional fiber ply is also referred to herein as a prepreg. A prepreg (a single unidirectional prepreg web, a single individual prepreg unidirectional tape (a single individual ply), or a multi-ply prepreg (i.e., comprising multiple unidirectional webs and / or multiple unidirectional tapes) is initially formed from unidirectionally aligned fibers / tapes that are not fully stretched, with the fibers / tapes having an elongation to break of at least 2.0%, typically at least 2.1%, or at least 2.2%, or at least 2.3%, or at least 2.4%, or at least 2.5%, or at least 2.6%, or at least 2.7%, or at least 2.8%, or at least 2.9%, or at least 3.0%, up to about 4.0% or 5.0% or more. The fibers / tapes that form the prepreg are then subjected to an axial stress that reduces the elongation to break and increases the toughness of the fibers / tapes, thereby improving the properties of composites formed from the fibers / tapes.

[0014] As used herein, the terms "orient" and "stretch" refer to the elongation of a fiber / yarn, and the terms are used interchangeably. When used in the context of "unidirectionally oriented fibers," the term "oriented" refers to the direction of fiber alignment, rather than the fiber's elongation. As used herein, a "fiber" is a long strand of material, such as a strand of polymeric material, whose length dimension is much greater than its transverse dimensions of width and thickness. Fibers are preferably long, continuous (but of clearly defined length) strands, rather than short sections of strands referred to in the art as "staple" or "staple fiber." A "strand," by conventional definition, is a single, thin, lengthy entity, such as a thread or fiber. The cross-section of a fiber for use herein may vary widely and may be circular, elliptical, flat, or oblong in cross-section. A strand may also be of irregular or regular multi-lobed cross-section, with one or more regular or irregular lobes protruding from the linear (axial) axis of the filament. Thus, the term "fiber" includes filaments, ribbons, strips, etc., having regular or irregular cross-sections. As used herein, the term "composite" refers to a combination of multiple fibers and / or tapes, typically with a polymeric binder material.

[0015] A single fiber may be formed from only one filament or from multiple filaments. Fibers formed from only one filament are referred to herein as either "single filament" or "monofilament" fibers, while fibers formed from multiple filaments are referred to herein as "multifilament" fibers. As defined herein, multifilament fibers preferably include from 2 to about 3000 filaments, more preferably from 2 to 1000 filaments, even more preferably from 30 to 500 filaments, even more preferably from 40 to 500 filaments, even more preferably from about 40 filaments to about 240 filaments, and most preferably from about 120 to about 240 filaments. Multifilament fibers are also often referred to in the art as fiber bundles or bundles of filaments. As used herein, "yarn" refers to a fiber bundle or bundle of filaments. The term "multifilament fiber" is defined as a single strand consisting of multiple filaments and is used interchangeably with "multifilament fiber." As used herein, the term "tenacity" refers to the tensile stress of an unstressed sample, expressed as force (grams) per unit of linear density (denier), as measured by ASTM D2256. The "initial modulus" of a fiber is the property of a material that describes its resistance to deformation. The term "tensile modulus" refers to the ratio of the change in tenacity, expressed in grams of force per denier (g / d), to the change in strain, expressed as a fraction of the original fiber length (in / in). The term "denier" is a unit of linear density, equal to the mass in grams per 9000 meters of fiber / yarn. In this regard, the fibers of the present disclosure may be of any suitable denier. For example, the fibers may have a denier of about 50 to about 5,000 denier, more preferably about 200 to about 5,000 denier, even more preferably about 650 to about 3,000 denier, and most preferably about 800 to about 1,500 denier. The selection is governed by considerations of ballistic effectiveness and cost. Finer fibers are more costly to manufacture and weave, but can produce greater ballistic effectiveness per unit weight. Like the fibers, the tape may be of any suitable denier, preferably having a denier of about 50 to about 30,000 denier, more preferably about 200 to 10,000 denier, even more preferably about 650 to about 2,000 denier, and most preferably about 800 to about 1,500 denier.

[0016] Processes for producing ultra-high molecular weight polyethylene (UHMW PE) fibers are known in the prior art, such as commonly owned U.S. Patent Nos. 7,846,363, 8,361,366, 8,444,898, 8,747,715, and 9,365,953, each of which is incorporated herein by reference to the extent consistent herewith. For example, in the processes of U.S. Patent Nos. 8,747,715 and 9,365,953, gel-spun yarns are made from ultra-high molecular weight polyethylene by adding a slurry of UHMW PE and a spinning solvent to an extruder and forming a liquid mixture from the slurry in the extruder, which is then passed through a heated vessel to form a homogeneous solution containing UHMW PE and the spinning solvent. The solution is then fed from a heated vessel to a spinneret to form a solution yarn, and the solution yarn from the spinneret is then drawn at a draw ratio of about 1.1:1 to about 30:1 to form a drawn solution yarn. The drawn solution yarn is then cooled to a temperature below the gel point of the UHMW PE polymer to form a gel yarn, and the gel yarn is then drawn one or more times in one or more stages. The spinning solvent is then removed from the gel yarn (by evaporation or extraction) to form a dry yarn, which is then drawn to form a partially oriented yarn and then further drawn in a post-draw operation to form a highly oriented yarn. In the context of this disclosure, the fibers useful herein produced from this method are either partially oriented fibers or highly oriented fibers. The break elongation values ​​of the fibers vary depending on the degree of orientation, generally ranging from 2.9% to 3.9% for highly oriented fibers and about 4.0% or greater for partially oriented fibers. For purposes of this disclosure, the required maximum or minimum thickness for the fibers (or tapes) forming the prepreg web / single discrete ply (UDT) / multi-ply fabric is There is no minimum elongation to break, but the fibers forming each prepreg web / single discrete ply (UDT) / multi-ply fabric have a preferred elongation to break of about 2.0% to about 3.9% after axial stretching in the compaction device, and a single pass through the compaction device under the conditions described herein will reduce the elongation to break by at least about 1% to about 5% or more.

[0017] In accordance with the disclosed method for improving composite tensile properties, a nonwoven web of unidirectionally oriented fibers (or tapes) is formed by placing continuous fibers / tapes in a parallel, parallel arrangement, most typically followed by coating the fibers / tapes with a polymeric binder material (also referred to herein as a "resin" or polymer "matrix") and allowing the binder material to dry so that the fibers / tapes are bonded together. This web of continuous, parallel, bonded fibers is referred to herein as a "prepreg web." As used herein, "Array" describes an ordered arrangement of fibers or threads, and "parallel array" describes an ordered, side-by-side, planar-oriented, parallel arrangement of fibers / tapes or threads. As used herein, the term "prepreg" more specifically refers to a combination of fibers or tapes connected to one another in the form of one or more fibrous plies or one or more fibrous webs, e.g., connected by adhesive but not yet consolidated / compressed in an isostatic press. When the ply / web comprises fibers, the fibers are coated / impregnated with a polymeric binder (as produced in FIG. 1); when the ply / web comprises tapes, the polymeric binder is optional. Tapes not coated with a polymeric binder are fused together under high heat and, optionally, pressure before being primed. Thus, a "prepreg" is a precursor used as a component of a subsequently formed material / article, and therefore for purposes of this disclosure, a "prepreg web" refers to a continuous, unidirectional web of nonwoven, parallel fibers / tapes coated with a binder / resin, as described above. A "prepreg unidirectional tape" is an individual single layer / ply of nonwoven parallel fibers (and therefore having a fixed length) that is cut from a prepreg web before being subjected to a compaction process in a compaction device (as in the process described in more detail below). Such individual single unidirectional nonwoven plies of parallel fibers / tapes having a fixed length are also referred to in the art as "unitapes," "UDs," or "UDTs," and are synonymous terms for "unidirectional tapes" and "unidirectional tape plies." In this regard, a "unidirectional tape" differs from the above-mentioned "tapes" in that a "tape" (fibrous or non-fibrous) is a fiber-like elongated body that can be grouped with other "tapes" in a parallel arrangement to form individual plies of "unidirectional tapes" only as multiple fibers (or combinations of tapes and fibers) can be grouped in a parallel arrangement to form a "unidirectional tape."As discussed above, a "multi-ply fabric" or "multi-ply prepreg fabric" is a combination of multiple individual nonwoven unidirectional tapes, each having a fixed length (e.g., cut from a continuous prepreg unidirectional fiber web), and adhered to one another by priming, such as by bonding them together by wet lamination under low pressure. Typically, but not necessarily, such multi-ply prepreg fabrics have an even number of plies, e.g., two, four, six, or eight, with each ply preferably rotated so that its longitudinal / axial fiber direction is at an angle to the adjacent ply. In this regard, each individual unidirectional tape ply has both an outer upper surface and an outer lower surface. The plies are overlapped such that the lower surface of one ply contacts the upper surface of the next individual ply, and the plies are rotated so that the longitudinal / axial fiber directions of adjacent plies are not equal. Such rotated unidirectional alignment is conventionally known in the art of ballistic composite armor and is described, for example, in U.S. Patent Nos. 4,457,985, 4,748,064, 4,916,000, 4,403,012, 4,623,574, and 4,737,402, all of which are incorporated herein by reference to the extent incompatible herewith. Most commonly, unidirectional plies are orthogonally cross-plied so that the fibers of all adjacent layers are oriented at angles between 0° and 90° relative to one another, although adjacent plies can be aligned at virtually any angle between about 0° and about 90° relative to the longitudinal / axial fiber direction of another ply. Methods for forming such multi-ply nonwoven prepreg fabrics are well known in the art and may be accomplished manually or automated with conventional stacking or cross-plying equipment, for example, using a cross-plying device such as taught in U.S. Patent No. 5,173,138, which is incorporated herein by reference to the extent consistent herewith, provided that the plies forming the multi-ply fabric are abutted under low pressure in accordance with the priming conditions described herein, since it is the priming step that forms the multi-ply structure.

[0018] When manufacturing prepregs, ideal fiber spreading is when individual fibers, or even individual filaments, are positioned next to each other in a single fiber plane, with a minimal amount, if any, of fibers overlapping each other to form a substantially unidirectional array of parallel fibers. Typically, a prepreg web or single prepreg unidirectional tape will contain about 3 to 55 fiber ends per inch (about 1.18 to about 21.65 ends per cm), more preferably about 5 to about 45 ends per inch (about 1.97 to about 17.72 ends per cm), even more preferably about 5 to about 20 ends per inch (about 1.97 to about 7.87 ends per cm), and most preferably about 6 to about 16 ends per inch (about 2.36 to about 6.30 ends per cm), with a typical fiber bundle (yarn) having about 30 to about 2000 individual filaments depending on the fiber type, with preferred polyethylene fiber bundles most typically having about 120 to about 240 individual filaments.

[0019] As noted above, an exemplary prepreg manufacturing process, as described above, is illustrated in FIG. 1. As illustrated, multifilament fiber bundles are fed from a clamp, directed through a collimating comb / heddle to align the fibers in the proper parallel arrangement, coated with a binder material / resin, and then pressed through a pair of spreader bars / rolls to spread the filaments of the fiber bundle and the binder. The binder is then dried or cured to complete the prepreg manufacturing process, which can then be rolled onto storage rolls for later use or cut into individual lengths. This method is merely exemplary and may be modified as desired. For example, the binder may actually be applied at any point in the manufacturing process, including any time before or after multiple fiber bundles are arranged in a parallel array. In such an alternative process, the fibers may instead be coated with the binder after passing through the spreader bars. In another alternative embodiment illustrated in Figure 2, an array of uncoated fibers (formed from fiber bundles aligned via heddle / comb, as described above) is coated with a polymer binder, such as in a conventional dip-and-squeeze coater, the excess binder is squeezed out, and the fibers are further spread by passing through additional rolls. The coated array is then combined with a release film / paper (e.g., silicone release paper), optionally dried, and sent for further processing, e.g., into a unidirectional tape of a desired length.

[0020] Following the formation of the prepreg web(s) and optional cutting of the web(s) into one or more individual unidirectional prepreg plies of desired length, the web(s) and / or one or more single unidirectional tape(s) are then primed. In a most preferred embodiment, multiple continuous fibrous webs or multiple single-plies of unidirectional tape are laminated (lightly bonded) together and primed to adhere to each other. "Priming" refers to lightly pressing the web or unidirectional tape ply / plies with or without heat between two platens at very low pressure (e.g., via a conventional flatbed laminator set at very low pressure) or by passing through the nip of a pair of laminating rolls by techniques well known in the art. Such priming steps include, among other things, wet laminating the plies together, and may optionally be performed in-line on a coater assembly (such as the combined roller illustrated in FIG. 2), as well as combinations of wet lamination and flatbed laminator / roll dual platens / nip. Such priming also includes pressing a single individual ply or a single unidirectional web, where such single individual ply or web is not attached to another individual ply or web. Regardless of the priming method, the pressure applied to the prepreg web or individual prepreg unidirectional tape ply(s) during priming is preferably less than 5 psi (0.0345 MPa), preferably between about 0.01 psi (68.9 Pa) and 2.0 psi (0.0138 MPa), more preferably between 0.01 psi (68.9 Pa) and 1.0 psi (0.00689 MPa), and most preferably between 0.1 psi (689 Pa) and 0.5 psi (0.00345 MPa). When priming is accomplished by heat, the heat is usually very low, i.e., in the range of just above room temperature (20-22°C) to about 50°C, preferably between about 20°C and about 50°C, more preferably between about 25°C and about 50°C, and most preferably between about 25°C and about 40°C, but more preferably between about 50°C and about 40°C. It may be carried out at elevated temperatures, for example, from about 90° C. to about 100° C. The prepreg web / unitape ply (or the combination of web / unitape plies that are not yet bonded together, which will become a multi-ply structure upon priming) is subjected to priming conditions for a short duration of from about 0.01 seconds to about 30 seconds, thereby pressing the fibers into a more ideal alignment.

[0021] Priming may also be performed using the same cross-ply equipment used in the prior art to consolidate multiple plies, such as the equipment of U.S. Patent No. 5,173,138. The equipment of U.S. Patent No. 5,173,138 includes means for laminating stacked plies, which is set at low temperatures and pressures and may be employed to prime the prepregs provided by the present disclosure. Priming can also be accomplished using any other conventional lamination equipment in which the web / UDT / web stack and / or UDT plies are treated according to the mild conditions described above.

[0022] Once primed, the web, single UDT ply, or multiple bonded webs and / or multiple bonded UDT plies (multi-ply prepreg web / fabric) are then heated and compressed in a suitable compression device under a pressure great enough to apply an axial extensional stress to each of the fibers forming the web or each UDT ply. Each of the fibers / tapes forming the web or each UDT ply is thereby stretched axially, i.e., stretched axially by being subjected to an axial compressive stress in the axial direction. If the prepreg is a multi-ply prepreg fabric or web, the multi-ply prepreg fabric / web is stretched multiaxially (e.g., a two-ply fabric is stretched biaxially) because the fibers / tapes of each web / UDT ply are stretched axially in the direction of each respective axis of the cross-plied fibers / tapes forming the individual component webs / UDT plies. Depending on the pressure, the axial extension stress is sufficient to increase the toughness of the component fibers by about 1% or more, preferably about 1% to about 5%, such that the elongation to break of each fiber is also reduced by at least about 1%, or about 1% to about 5%, resulting in greater axial extension.

[0023] The temperature to which the single prepreg UDT ply, multi-ply fabric, and / or web(s) are heated will vary depending on the fiber type and polymer binder type (if any) that form the prepreg. In this regard, suitable fibers are synthetic thermoplastic fibers having an axial (linear) thermal conductivity, when measured at 23°C (room temperature), of at least 0.1 W / (m·K) to about 300 W / (m·K), more preferably about 0.1 W / (m·K) to about 125 W / (m·K), even more preferably about 0.1 W / (m·K) to about 105 W / (m·K), still more preferably about 50 W / (m·K) to about 105 W / (m·K), and most preferably about 80 W / (m·K) to about 105 W / (m·K), when measured by conventional known methods such as the guarded hot plate method according to ASTM C-177 (measuring up to 100 W / (m·K)) or the guarded heat flow sensor method according to ASTM C518 (measuring values ​​greater than 100 W / (m·K)). These polymer types have both a softening temperature (Vicat softening temperature as determined by ASTM D1525 / ISO306) and a melting temperature. Suitable synthetic thermoplastic polymers include, but are not limited to, polyolefin fibers such as extended-chain polyethylene and extended-chain polypropylene fibers, polyester fibers including polyethylene terephthalate fibers, nylon fibers, polyvinyl acetate fibers, and polyacrylonitrile fibers. The melting points of all thermoplastic fiber types except UHMW PE can be determined by ASTM D7138. UHMW PE has such a high molecular weight that its melting point is determined by a different method, ASTM F2625. For example, UHMW PE fibers with a c-axis orientation function of at least 0.96 and an intrinsic viscosity of about 7 dl / g to 40 dl / g, as measured in decalin at 135°C by ASTM D1601-99, have a Vicat softening temperature of 80°C and a melting temperature of about 136°C as measured by ASTM F2625. Therefore, the UHMW PE fibers are heated to a temperature of about 80°C to about 136°C after priming and in preparation for compression, preferably about 90°C to about 136°C. The polyethylene fibers are heated to a temperature of 0°C to about 136°C, or about 115°C to about 136°C. Lower molecular weight polyethylene fibers will have lower softening and melting temperatures, which can be easily determined by one skilled in the art. Fiber types with axial (linear) thermal conductivities of less than 0.1 W / (m·K), such as aramid, graphite (carbon), glass, quartz, and basalt fibers, do not exhibit improved toughness but may still benefit from similar processing. For example, composite types containing any primed fiber and binder type will benefit from the improved compaction and reduced air voids within the composite caused by passing the composite through a compaction conveyor.

[0024] Methods for heating single UDT, multi-ply prepreg fabric(s) / web(s) are conventional in the art and include passing the prepreg web or cut unidirectional plies through one or more ovens, preferably forced air convection ovens such as those described in U.S. Patent No. 7,344,668, with oven residence times selected within the skill of one in the art and similarly heated with heated platens. Alternatively, the prepreg may be heated by the compaction equipment, if the compaction equipment has heating capabilities, which is most preferred.

[0025] Once heated to an appropriate temperature (provided it is preheated), the primed prepreg web(s) / individual plies / multi-ply fabric (each of which is also referred to herein as the "material" or "priming material" passing through the compaction device) is compacted in a compaction device capable of applying significant uniform pressure to the prepreg material, preferably also conveying the composite through the compaction device. Additionally, if a thicker material is desired depending on the desired end use, multiple such lightly bonded primed laminates of unidirectional tape may be coextensively combined and compacted together in the same compaction device. It is specifically contemplated that the compaction device is a compaction conveyor, most specifically a double belt press or a steel belt press. Most preferably, the double belt press is an isobaric double belt press or an isobaric steel belt press when the belt comprises steel. A double belt press is a type of compaction conveyor that includes two pressure-applying elements, typically the steel belts mentioned above. The material is compressed between pressure-applying elements, whereby the pressure simultaneously subjects the composite to an axial extensional stress while the composite is simultaneously conveyed through a compression device.

[0026] Suitable double belt presses are commercially available from, for example, Hymmen International Inc. (Grand Rapids, Michigan), Dieffenbacher GmbH & Co. (Eppingen, Germany), Held Technologie GmbH (Trossingen, Germany), Sandvik Process Systems LLC (Totowa, New Jersey), and Siempelkamp LP (Marietta, Georgia), among others.

[0027] An exemplary isobaric double belt press 2 is shown in Figure 3. Referring to Figure 3, such a suitable double belt press 2 is shown and includes a first belt 10, a second belt 12, a first plurality of rollers 4 supporting the first belt 10, a second plurality of rollers 6 supporting the second belt 12, and a temperature unit 8. As shown, the first plurality of rollers 4 rotate in a clockwise direction and are positioned above the second plurality of rollers 6. Conversely, the second plurality of rollers 6 rotate in a counterclockwise direction and, in combination with the clockwise rotation of the first plurality of rollers 4, advance the primed material 14 through the press 2.

[0028] As the primed material 14 enters the isostatic press 2, the first belt 10 and the second belt 12 The belt 12 is configured to apply a continuous pressure of about 45 bar (about 600 psi) to about 80 bar (1100 psi) to the primed material 14 as it advances through the isostatic press 2. The continuous high pressure is applied uniformly to the primed material 14, subjecting the fibers to an axial extensional stress. The axial extensional stress axially stretches or elongates each of the fibers in their respective axial fiber direction (i.e., the length direction in which the fibers / tapes are oriented / stretched into each component unidirectional web / UDT ply). This biaxial stretching increases the material's properties overall in both directions. As noted above, if the pressed primed material is only a single individual UDT ply or unidirectional web, the extensional stress is uniaxial. If the material being primed is a bonded combination of multiple cross-plied UDT plies or cross-plied multi-web prepregs, the extensional stress is biaxial or multiaxial, with the axial stress applied individually to the axial fiber direction of each web / UDT ply. The residence time of the primed material in the press is continued for a time sufficient to increase the tenacity of each fiber by at least about 1%, preferably 1% to about 5%, and the elongation at break of each fiber is reduced by at least 1%, preferably 1% to about 5%, achieved under the pressing conditions described herein.

[0029] In addition to being pressed by the first belt 10 and the second belt 12, the material 14 passes through a temperature unit 8, which includes a heating section 8a and a cooling section 8b. Thus, the material 14 is subjected to continuous high pressure from the first belt 10 and the second belt 12, while being heated (first) and then cooled (second). By actively cooling the fibers in this manner while the material is under pressure in the press, rather than allowing the fibers to cool naturally to room temperature as they exit the press, the elongated fiber structure is maintained; whereas, as the fabric cools naturally, the warm fibers exhibit some contraction, reversing some of the fiber elongation and thereby at least partially reversing the improved fiber properties. Importantly, the pressure exerted by the press on the material constrains the fibers, thereby preventing the fiber contraction that would normally occur, which involves a restoration of the fiber molecular structure to a more disordered state and reduces the tensile properties of the fiber / fibrous material.

[0030] This combination of intense pressure with both heating and cooling results in a high degree of compaction and reduction of air voids and other voids within the pressed material, thereby providing a smooth, substantially uniform surface that is more resistant to corrosive and degrading conditions, including resistance to environmental conditions such as gasoline, salt water, humidity, etc. These are believed to reduce the material properties of such fibrous composites compared to pressed material that is simply cooled naturally to room temperature rather than being actively cooled in a cooling zone.

[0031] In a preferred embodiment, the material passing through the press is maintained under continuous and constant axial tension by a tensioning device, which may be a storage roll (not shown) around which the pressed material is wound, which is actively rotated to apply tension to the material as it passes through the press. In this embodiment, the primed material is typically a single web or a multi-ply web. Rear tension may also be applied by providing a roll of material to the press from a supply roll (i.e., a roll that stores rolled-up primed material fed to the press, not shown) that is actively rolled (thereby unwinding the primed material for passage through the press) at a slower speed than the storage roll, or by limiting the roll speed of the supply roll to a speed slower than the speed at which the material passes through the press, with or without the presence of a storage roll, such as by equipping the supply roll with a brake or another type of tensioner, such as a disk-type tensioner, a pin-type tensioner, a dancer arm tensioner, or the like, as determined by one skilled in the art. Again, the primed material is typically a single web or a multi-ply web. Typically, the overall axial tension in each fiber is about 10% as the material passes through the press. It is desirable for the tension to be in the range of 0 to about 1000 grams, more preferably in the range of about 200 to about 800 grams. It is preferable for the tension to be continuous and constant, particularly when the material passing through the compression conveyor is in the form of a continuous web of material, whether the web is compressed or contains a single unidirectional array of continuous fibers, or is a continuous multi-ply web formed from multiple primed fiber plies.

[0032] In another embodiment, when the material passing through the press comprises two or more individual fiber plies, such as one or more two-ply prepreg fabrics, simultaneous axial and transverse axial tension can be exerted on the material by mounting the material on an axial or biaxial tension frame such as disclosed in U.S. Pat. No. 3,153,812, which is incorporated herein by reference to the extent consistent therewith.

[0033] With respect to residence time in the press, in a typical isobaric belt press having a 1.5-meter heating zone immediately followed by a 1.0-meter cooling zone, the material moves through the press at a speed of about 2 meters / minute to about 10 meters / minute, most preferably about 4 meters / minute to about 8 meters / minute. In an exemplary process in which the material passes through the press at about 6 meters / minute, the residence time in the heating zone is about 15 seconds and the residence time in the cooling zone is about 10 seconds. When the material passes through the press at these speeds and with an applied pressure of about 45 bar (about 600 psi) to about 80 bar (1100 psi), and the other processing conditions described herein include subsequent cooling to room temperature under continuous, maintained constant pressure as described herein, the tenacity of each fiber will increase by at least about 1% and the elongation to break of each fiber will decrease by at least 1%. Additionally, the primed and compressed material exhibits higher shear strength properties and a lower coefficient of friction compared to a comparative material that was not processed through an isopressure double belt press.

[0034] In a most preferred embodiment, the primed material is subjected to an axial extensional stress having a magnitude of at least about 1 pound per lineal inch (PLI) to about 30-PLI at a temperature of about 90°C to about 125°C. Passing through at a sufficient speed is expected to result in an increase in tensile properties of at least 5% for fibrous webs containing at least ultra-high molecular weight polyethylene fibers.

[0035] When the primed material being conveyed through the press is in the form of a continuous single-ply or multi-ply web, the web is preferably provided in roll form from a first storage roll, as shown in FIGS. 1 and 4, and the web is preferably similarly wound onto a second storage roll after exiting the press. During conveyance through the press from such first storage roll to such second storage roll, the storage roll may be actively rotated to apply tension to the web as it is conveyed through the press. Mechanisms and methods for rotating such storage rolls are known in the art and would be readily configured by one of ordinary skill in the art. When such tension is applied to the web, the tension is preferably in the range of about 2 Newtons / cm to about 50 Newtons / cm, or about 5 Newtons / cm to about 50 Newtons / cm, with a tension of 2 N / cm to about 25 N / cm being more preferred, and a tension of 2 N / cm to about 13 N / cm being most preferred.

[0036] When one or more multi-ply fabrics (e.g., multiple two-ply fabrics) or one or more multi-ply webs (e.g., multiple two-ply webs) pass through a press, the significant pressure of the press completely consolidates the fabrics / webs into a unitary composite, a monolithic layer or single, monolithic web, where the component layers / webs are no longer separable from one another (at least without destroying the removed material). They are converted into a single, consolidated, monolithic multi-ply structure. The total number of unidirectional tapes or webs incorporated into the composite is The number of plies will also depend on the end use of the composite and will be readily determined by one of ordinary skill in the art. For example, in armor applications, a composite armor article may contain from 2 to about 100 two-ply UDTs, more preferably from about 2 to about 85 two-ply UDTs, and most preferably from about 2 to about 65 two-ply UDTs. In the context of the present disclosure, these multi-ply materials are preferably formed by cutting multiple individual lengths from a multi-ply web compressed in the compaction apparatus of the present disclosure, followed by another consolidation step, such as sewing or conventional fibrous composite molding techniques. The greater the total number of plies, the greater the ballistic resistance, but also the greater the weight. Minimum levels of ballistic resistance for body armor for military use are classified by the National Institute of Justice (NIJ) Threat Levels, as is well known in the art. For example, The two-ply fabric (also referred to herein as the "fibrous layer") is approximately 60 g / m 2 ~Approx. 130g / m 2 and has an areal density of 1.0 lb / ft 2 (psf)(4.88kg / m 2 To produce an armor article having an areal density of 1000 kPa (ksm), a total of about 38 to about 81 two-ply fabrics must be combined in a press.

[0037] The areal density of a single web ply or a single UDT ply, or multiple UDT / multi-web materials passing through an isopressure press, is not intended to be strictly limited, but each single unidirectional web ply or single unidirectional tape ply generally has an areal density of about 10 g / m 2 ~about 95g / m 2 , or about 15 g / m 2 ~about 95g / m 2 or approximately 30 g / m 2 ~about 95g / m 2The multi-unit or multi-web laminate that passes through the press preferably contains 2 to 8 total plies (webs and / or unitapes), each ply comprising fibers coated with a polymeric binder / matrix that is preferably a very flexible, low modulus polymer, such as an elastomeric and / or thermoplastic polymeric binder having an initial tensile modulus of less than 6,000 psi, more preferably 4,000 psi or less, and most preferably 2,000 psi or less.

[0038] As mentioned above, preferred fiber types herein are synthetic thermoplastic fibers that are thermally conductive and have a thermal conductivity of at least 0.1 W / (m·K). Such fibers typically exhibit both a softening temperature (Vicat softening temperature as determined by ASTM D1525 / ISO 306) and a melting temperature. These include polyolefin fibers such as extended chain polyethylene and extended chain polypropylene fibers, polyester fibers including polyethylene terephthalate fibers, nylon fibers, fiber types such as polyvinyl acetate fibers and polyacrylonitrile fibers, and polyvinyl acetate and polyacrylonitrile fibers. Also useful are melt-spun polyethylene fibers, such as those taught in U.S. Pat. No. 7,935,283, which is incorporated herein by reference to the extent consistent therewith. Polyethylene tapes, for example, as described in commonly owned U.S. Pat. No. 8,263,119, are also useful. US Patent Nos. 8,697,220, 8,685,519, 8,852,714, 8,906,485, 9,138,961, and 9,291,440 are also useful, each of which is incorporated herein by reference to the extent consistent with the present specification. In this regard, the term "tape" refers to a flat, narrow, monolithic strip of material having a length greater than its width and an average cross-sectional aspect ratio of at least about 3:1, i.e., the ratio of the maximum dimension to the minimum dimension of the average cross-section of the tape article throughout its length. Tapes may be fibrous or non-fibrous materials, with fibrous materials containing one or more filaments, such as those described in the above-referenced patents. Non-fibrous tape materials may be formed, for example, from strips of polymer formed by slicing a polymer film. Tapes are distinguished from fibers having a flat or rectangular cross section in that the fibers have an aspect ratio of less than 3:1, and typically have an aspect ratio of 1:1 to 2:1.

[0039] Aramid fiber, M5® fiber, graphite (), which are not melt processable and exhibit very low thermal conductivity (<0.1 W / (m·K)) when measured at 23°C (room temperature), are also used. Fiber types such as carbon, glass, quartz, and basalt fibers may also benefit from similar processing, but will not see the same toughness improvements as other fiber / tape types, i.e., they will experience little or no elongation when passed through the press, thereby not achieving at least a 5% increase in toughness and at least a 5% decrease in elongation at break, but may benefit from the improved compaction as discussed above.

[0040] In the case of polyethylene, preferred fibers are extended chain polyethylene having a weight average molecular weight of at least 300,000, preferably at least 5,000,000, more preferably at least 1,000,000, and even more preferably 2 to 5,000,000. Polyethylenes having a weight average molecular weight of at least 1,000,000 are generally referred to as ultra-high molecular weight polyethylene. Such extended chain polyethylene (ECPE) fibers are The fibers may be grown by a solution spinning process such as described in U.S. Pat. Nos. 4,137,394 or 4,356,138, which are incorporated herein by reference, or by any of the methods described in U.S. Pat. Nos. 4,413,110, 4,536,536, 4,551,296, 4,663,101, 5,006,390, 5,032,338, 5,578,374, 5,736,244, 5,741,451, 5,958, The fibers may also be spun from solution to form gel structures, such as those described in US Pat. Nos. 5,972,498, 6,448,359, 6,746,975, 6,969,553, 7,078,099, 7,344,668, 7,846,363, 8,361,366, 8,444,898, 8,747,715, and 9,365,953, all of which are incorporated herein by reference. A particularly preferred fiber type is Honeywell's The polyethylene fibers may be any of those sold under the trademark SPECTRA® by Royal DSM NVC Corporation, Inc. SPECTRA® fibers are well known in the art. Other useful polyethylene fiber types also include DYNEEMA® UHMW PE yarns, available from Royal DSM NVC Corporation, Heerlen, The Netherlands. The UHMW PE manufacturing methods of U.S. Patent Nos. 8,747,715 and 9,365,953, described in more detail above, are merely exemplary.

[0041] Particularly preferred methods for forming UHMW PE fibers are those capable of producing UHMW PE fibers having a tenacity of at least 35 g / denier, most preferably multifilament fibers. Most preferred processes include those described in commonly owned U.S. Patent Nos. 7,846,363, 8,361,366, 8,444,898, 8,747,715, and 9,365,953, the disclosures of which are incorporated herein by reference to the extent consistent with this specification. These processes are capable of producing very high tenacity UHMW PE fibers. Such processes are referred to as "gel spinning" processes, also referred to as "solution spinning," in which a solution of ultra-high molecular weight polyethylene and a solvent are formed, followed by extrusion of the solution through a multi-orifice spinneret to form solution filaments, quenching the solution filaments into gel filaments, and extracting the solvent to form dry filaments. These dry filaments are then gathered into bundles, referred to in the art as either fibers or yarns. The fiber / yarn can then be stretched (drawn) to its maximum elongation limit to increase toughness. As the fiber is drawn, the elongation to break decreases. This means that when drawn, the degree to which it can be stretched is essentially reduced further.

[0042] Other suitable fiber types, as well as methods for forming suitable polyethylene tapes, are well known in the art. The drawing of all fibers / tapes useful herein should be limited to the point where their elongation to break is still at least 2.0%, but drawing should be such that a minimum tenacity of at least 7 g / denier, with a preferred tenacity of at least about 150 g / denier, is achieved. The tensile modulus should be limited to a point sufficient to achieve fibers having a tensile modulus, preferably an energy-to-break of at least about 8 J / g, each as measured by ASTM D2256. However, the composites of the present disclosure are intended for high performance applications such as ballistic materials or impact resistant textiles, and therefore it is desirable for the fibers forming each unidirectional fibrous ply (web and UDT) to be substantially greater than 7 g / denier. In this regard, it is preferred that each of the fibers forming the fibrous ply have a tenacity of greater than 25 g / denier, more preferably from about 28 g / denier to about 50 g / denier, even more preferably from about 33 g / denier to about 50 g / denier, even more preferably 39 g / denier or greater, even more preferably at least 39 g / denier to about 50 g / denier, even more preferably 40 g / denier or greater, even more preferably 43 g / denier or greater, or at least 43.5 g / denier, even more preferably from about 45 g / denier to about 50 g / denier, even more preferably at least 45 g / denier, before being subjected to an axial tensile stress (e.g., a double belt press). After being subjected to an axial tensile stress (e.g., a double belt press), the fibers will have a tenacity that is at least 1%, preferably at least 2%, more preferably at least 5%, even more preferably 10% or greater, and most preferably at least 15% greater than the tenacity of the original fibers before being subjected to the axial tensile stress. These axially stretched fibers of the final compressed composite will have a tenacity greater than about 27 g / denier, more preferably from about 28 g / denier to about 60 g / denier, even more preferably from about 33 g / denier to about 60 g / denier, even more preferably 39 g / denier or greater, even more preferably at least 39 g / denier to about 60 g / denier, even more preferably 40 g / denier or greater, even more preferably 43 g / denier or greater, or at least 43.5 g / denier, even more preferably from about 45 g / denier to about 60 g / denier, even more preferably at least 45 g / denier, at least about 48 g / denier, at least about 50 g / denier, at least about 55 g / denier, or at least about 60 g / denier, with similar tenacity ranges for the fibrous tapes.Fibers having these tenacity ranges can be achieved by conventional drawing without subjecting the fiber to axial stretching in a compaction device, however, greater results are achieved when the peak tenacity of the fiber / tape is achieved by a combination of drawing and compressive stretching (i.e., axial stretching in a compaction device) rather than by drawing alone for the reasons discussed above.

[0043] The fibers of the present disclosure have a higher denier before compression axial stretching compared to after compression axial stretching. In this regard, the pre-compression stretched fibers preferably have a denier of 2.0 to 10.0, more preferably about 2.0 dpf to about 9.0 dpf, even more preferably about 2.0 dpf to about 8.0 dpf, and most preferably about 2.0 dpf to about 7.0 dpf. The post-compression stretched fibers have a denier per filament reduced by at least 1.0, thereby having a denier per filament having a dpf (depending on the starting dpf) of about 1.0 to about 9.0 dpf, more preferably about 1.0 dpf to about 8.0 dpf, even more preferably about 1.0 dpf to about 7.0 dpf, even more preferably about 1.0 dpf to about 6.0 dpf, even more preferably about 1.0 dpf to about 5.0 dpf, and most preferably about 1.0 dpf to about 4.0 dpf.

[0044] The polymeric binder material, as mentioned above, is referred to in the art as a polymeric "matrix" material or "resin," and is a material that bonds to the fibers (and tapes, if present) either by its inherent adhesive properties or after being subjected to well-known conditions of heat and / or pressure. As used herein, a "polymeric" binder or matrix material is preferably, but not necessarily thermoplastic, and includes resins and rubbers. When present, the polymeric binder / matrix material partially or substantially coats the individual fibers / tapes, and preferably substantially coats each of the individual filaments / fibers / tapes to form the prepreg.

[0045] Suitable polymeric binder materials include both low tensile modulus, i.e., elastomeric materials, and high tensile modulus, rigid materials. As used throughout this specification, the term tensile modulus refers to the coefficient of elasticity of the polymeric binder material as measured by ASTM D638. Low or high modulus binders may include a variety of polymeric and non-polymeric materials. For purposes of this disclosure, a low modulus elastomeric material has a tensile modulus measured at about 6,000 psi (41.4 MPa) or less according to the ASTM D638 testing procedure. A low modulus polymer is preferably an elastomer having a tensile modulus of about 4,000 psi (27.6 MPa) or less, more preferably about 2400 psi (16.5 MPa) or less, even more preferably 1200 psi (8.23 MPa) or less, and most preferably about 500 psi (3.45 MPa) or less. The glass transition temperature (T g ) is preferably less than about 0°C, more preferably less than about -40°C, and most preferably less than about -50°C. The low modulus elastomeric material also has a preferred elongation at break of at least about 50%, more preferably at least about 100%, and most preferably at least about 300%. Whether low modulus or high modulus material, the polymeric binder may also contain fillers such as carbon black or silica, may be oil extended, or may be vulcanized with sulfur, peroxide, metal oxide, or radiation curing systems as are well known in the art.

[0046] A wide variety of materials and formulations may be used as low modulus polymer binders. Representative examples include polybutadiene, polyisoprene, natural rubber, ethylene-propylene copolymers, ethylene-propylene-diene terpolymers, polysulfide polymers, polyurethane elastomers, chlorosulfonated polyethylene, polychloroprene, plasticized polyvinyl chloride, butadiene-acrylonitrile elastomers, poly(isobutylene-co-isoprene), polyacrylates, polyesters, polyethers, fluoroelastomers, silicone elastomers, copolymers of ethylene, polyamides (useful with some fiber types), acrylonitrile butadiene styrene, polycarbonates, and combinations thereof, as well as other low modulus polymers and copolymers that can be cured below the melting point of the fibers. Also useful are blends of different elastomeric materials, or blends of one or more thermoplastics with elastomeric materials.

[0047] Particularly useful are block copolymers of conjugated dienes and vinyl aromatic monomers. Butadiene and isoprene are preferred conjugated diene elastomers. Styrene, vinyl toluene, and t-butyl styrene are preferred conjugated aromatic monomers. Block copolymers incorporating polyisoprene may be hydrogenated to produce thermoplastic elastomers with saturated hydrocarbon elastomer segments. Polymers include simple triblock copolymers of type ABA, type (AB) n (n=2~10) multi-block copolymer, or type R-(BA) x(x=3-150) where A is a block from a polyvinyl aromatic monomer and B is a block from a conjugated diene elastomer. Many of these polymers are commercially produced by Kraton Polymers (Houston, Texas) and are described in the report "Kraton Thermoplastic Rubber," SC-68-81. Also useful are resin dispersions of styrene-isoprene-styrene (SIS) block copolymers, sold under the PRINLIN® trademark by Henkel Technologies, based in Düsseldorf, Germany. Traditional low-modulus polymer binder polymers employed in ballistic composites include polystyrene-polyisoprene-polystyrene block copolymers, sold under the trademark KRATON®, commercially produced by Kraton Polymers.

[0048] A low modulus polymeric binder material is preferred for forming a flexible impact resistant material, while a high modulus High modulus polymeric binder materials are preferred for forming rigid impact resistant articles. High modulus rigid materials generally have an initial tensile modulus greater than 6,000 psi. Useful high modulus rigid polymeric binder materials include polyurethanes (both ether and ester based), epoxies, polyacrylates, phenolic / polyvinyl butyral (P VB) polymers, vinyl ester polymers, styrene-butadiene block copolymers, and mixtures of polymers such as vinyl ester and diallyl phthalate, or phenol formaldehyde and polyvinyl butyral. Particularly useful rigid polymer binder materials are thermosetting polymers that are soluble in carbon-carbon saturated solvents such as methyl ethyl ketone and have a viscosity of at least about 1×10 as measured by ASTM D638. 6It has a high tensile modulus upon cure at 100 psi (6895 MPa).Particularly useful rigid polymeric binder materials are those described in U.S. Patent No. 6,642,159, which is incorporated herein by reference.

[0049] Most specifically preferred herein are polar resins or polar polymers, especially polyurethanes ranging from about 2,000 psi (13.79 MPa) to about 8,000 psi (55.16 MPa) in tensile modulus, both soft and hard. Preferred polyurethanes are applied as aqueous polyurethane dispersions, most preferably, but not necessarily, cosolvent-free. These include aqueous anionic polyurethane dispersions, aqueous cationic polyurethane dispersions, and aqueous nonionic polyurethane dispersions. Particularly preferred are aqueous anionic polyurethane dispersions, aqueous aliphatic polyurethane dispersions, and most preferred are aqueous anionic, aliphatic polyurethane dispersions, all of which are preferably cosolvent-free dispersions. These include aqueous anionic polyester-based polyurethane dispersions, aqueous aliphatic polyester-based polyurethane dispersions, and aqueous anionic, aliphatic polyester-based polyurethane dispersions, all of which are preferably cosolvent-free dispersions. These also include aqueous anionic polyether polyurethane dispersions, aqueous aliphatic polyether-based polyurethane dispersions, and aqueous anionic, aliphatic polyether-based polyurethane dispersions, all of which are preferably cosolvent-free dispersions. Also preferred are all corresponding variations of aqueous cationic and aqueous nonionic dispersions (polyester-based, aliphatic polyester-based, polyether-based, aliphatic polyether-based, etc.). Most preferred are aliphatic polyurethane dispersions having a modulus at 100% elongation of about 700 psi or greater, with a particularly preferred range of 700 psi to about 3000 psi. Even more preferred are aliphatic polyurethane dispersions having a modulus at 100% elongation of about 1000 psi or greater, even more preferably about 1100 psi or greater. Most preferred are aliphatic polyether-based anionic polyurethane dispersions having a modulus at 100% elongation of 1000 psi or greater, more preferably 1100 psi or greater.

[0050] The total weight of the binder forming the prepreg / composite material of the present disclosure is preferably about 2% to about 50% by weight, more preferably about 5% to about 30% by weight, more preferably about 7% to about 20% by weight, and most preferably about 13% to about 16% by weight of the fiber plus the weight of the binder. The most preferred amount of binder varies depending on the end-use requirements. The amount of binder also depends on whether the component plies are formed from fiber or tape. When plies are formed from tape, a smaller amount of binder is generally used because the binder remains on the surface. Typically, when two plies formed from tape are bonded together, the binder content is less than 10% by weight of the combined plies, most typically about 4% to about 8% by weight of the combined plies; optionally, adjacent tapes are fused together and no binder is present between the tapes.

[0051] Methods for applying polymeric binder materials to fibers to form binder-impregnated prepregs are well known and readily determined by those skilled in the art. The term "impregnated" is used herein to refer to the polymeric coating being "embedded," "coated," or "bonded." is considered synonymous with "differently applied" (i.e., the binder material is dispersed throughout the web / UDT ply and not merely present on the surface of the web / UDT ply). Any suitable application method may be used to apply the polymeric binder material, and the specific use of terms such as "coated" is not intended to limit the manner in which the filaments / fibers / tapes are applied. Useful methods include, for example, spraying, extruding, or roll coating the polymer or polymer solution onto the fibers / tapes, as well as conveying the fibers / tapes through a molten polymer or polymer solution. The most preferred method is one in which each individual fiber / tape is substantially coated or encapsulated, covering the entire or substantially the entire surface area of ​​the fibers / tapes with the polymeric binder material.

[0052] After a composite article is produced by passing through a compression device (e.g., a compression conveyor / press), or alternatively, before one or more webs and / or one or more unidirectional tape plies pass through the compression device, one or more thermoplastic polymer films may be attached to the outermost surface of the webs / plies of the material (before pressing) or the compressed composite (after pressing). When attached to the webs / plies before pressing, the film is simply placed on the outer surface of the outermost web / ply, and the film is fused to the webs / plies as the combination passes through the press, as described in the examples below. When attached after the web(s) / UDT ply(s) / UDT plies are compressed in the press, the film can be laminated onto the compressed composite according to conventional thermal lamination conditions in the art. Typically, lamination is performed by positioning individual layers on top of each other under sufficient heat and pressure to bond the layers into a unitary structure. Lamination may be carried out at a temperature ranging from about 95° C. to about 175° C., preferably from about 105° C. to about 175° C., at a pressure of about 5 psig (0.034 MPa) to about 100 psig (0.69 MPa), for a time period of about 5 seconds to about 36 hours, preferably about 30 seconds to about 24 hours. Alternatively, such thermoplastic films may be bonded to the composite surface with hot glue or hot melt fibers, as will be understood by those skilled in the art.

[0053] Suitable polymers for the thermoplastic polymer layer include, but are not limited to, polyolefins, polyamides, polyesters (especially polyethylene terephthalate (PET) and PET copolymers), polyurethanes, vinyl polymers, ethylene vinyl alcohol copolymers, ethylene octane copolymers, acrylonitrile copolymers, acrylic polymers, vinyl polymers, polycarbonates, polystyrenes, fluoropolymers, and the like, as well as copolymers containing ethylene vinyl acetate (EVA) and ethylene acrylic acid, and mixtures thereof. Also useful are natural and synthetic rubber polymers. Of these, polyolefin and polyamide layers are preferred. A preferred polyolefin is polyethylene. Non-limiting examples of useful polyethylenes include low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), and linear low density polyethylene (LDPE). Polyethylenes include medium density polyethylene (MDPE), linear medium density polyethylene (LMDPE), linear very-low density polyethylene (VLDPE), linear ultra-low density polyethylene (ULDPE), high density polyethylene (HDPE), and copolymers and mixtures thereof. Examples include SPUNFAB® polyamide webs (registered trademarks of Keuchel Associates, Inc.) available from Spunfab, Ltd., Cuyahoga Falls, Ohio, and THERMOPLAST™ and HELIOPLAST™ webs, netting, and films available from Protechnic SA, Cernay, France.

[0054] The optional thermoplastic polymer layer is preferably very thin, from about 1 μm to about 250 μm, more preferably Preferably, the layer thickness is from about 5 μm to about 25 μm, and most preferably from about 5 μm to about 9 μm. Discontinuous webs, such as SPUNFAB® nonwoven webs, preferably have a thickness of 6 g / m 2 It is understood that while such thickness is preferred, other thicknesses may be manufactured to meet particular needs and are within the scope of this disclosure.

[0055] Finally, a particular advantage is achieved when the fibers / tapes forming the prepreg web / UDT ply / multi-ply fabric have an initial elongation to break of 3.0% or more, or more preferably, 4.0% or more before being subjected to the axial extension stress and 2.0% to 3.9% after the axial extension stress. For example, UHMW PE fibers useful herein can be produced according to the method of U.S. Pat. No. 9,365,953, but post-stretching of the fiber(s) is limited to maintaining a break elongation greater than 4.0%, and these fibers are stretched in a post-stretching operation (i.e., a stretching operation after the spinning solvent has been removed / evaporated and the fibers have been dried, as defined in U.S. Pat. No. 9,365,953) while the fiber break elongation is greater than 4.0%. After post-stretching, the fibers are further stretched axially in a compression device (e.g., a double-belt press) as described herein, thereby further reducing the break elongation by at least 1%. The final elongation to break depends on the starting elongation to break before passing through the compactor. Preferred fibers have a final elongation to break of 2.0% to 3.9%, preferably 2.9% to 3.9%, by being axially stretched in the compactor, and the elongation to break of each fiber is reduced by at least 1% by passing through the compactor under the above conditions. If the post-stretching of the fibers forming the prepreg web / UDT ply / multi-ply fabric is terminated early so that their elongation to break remains above 4.0% upon entering the compactor, significant minimization of yield loss problems associated with maximum fiber stretching should be achieved (e.g., according to the method described in U.S. Pat. No. 9,365,953 or other methods for forming high-tenacity UHMW PE fibers). However, fibers / tapes with any elongation to break may be compressed and axially stretched according to the process of the present disclosure, and the elongation to break of each fiber will be reduced by at least 1%, regardless of its elongation to break / elongation upon entry into the compactor.

[0056] While the present technology has been particularly shown and described with reference to preferred embodiments, it will be readily apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the present disclosure. The claims are intended to be construed to cover the disclosed embodiments, their alternatives as described above, and all equivalents thereof. For example, rather than attaching two unidirectional squares / rectangles to each other by priming them before passing through a press (e.g., each cut from a unidirectional fibrous web and then coextensively laminated surface-to-surface, followed by priming), an alternative method may be used in which two unidirectional webs are first formed into a two-ply web, such as a two-ply 0° / 90° web, by priming two unidirectional webs together as shown in FIG. 4. In this embodiment, a first unidirectional prepreg web of 0° axially oriented fibers is formed (e.g., as described in more detail above, such as in the method illustrated in FIG. 1 ) (Step 1), followed by converting the web of axially oriented fibers to a web of fibers oriented at 90° (or any other angle, e.g., ±45°) by cutting the 0° web into sections, rotating the cut sections so that the fibers are oriented at the desired angle, and then taping the cut sections together to form a new web as shown in FIG. 4 (Step 2). This new web, e.g., a 90° web as shown in FIG. 4 , can then be adjacent to another 0° unidirectional prepreg web (Step 3), such as by wet lamination, optionally with intermediate and / or surface polymer films, and with existing resin / binder coatings on the fibers or optional polymer films that act as adhesives to help adhere the webs to each other. Thereafter, a two-ply web or multiple two-ply webs (e.g., multiple two-ply, 0° webs) can be formed. 4. The resulting web (with wide, coextensively adjacent surfaces) is passed through a compression device as described above (step not shown in FIG. 4).

[0057] The following non-limiting examples serve to illustrate preferred embodiments of the present disclosure.

[0058] Comparative Examples 1 to 13 A continuous four-ply composite web was fabricated from SPECTRA® ultra-high molecular weight polyethylene filament bundles / fibers (1332 denier unidirectional filament bundles; 5.5 denier per filament (dpf); approximately 36.6 grams per denier (g / d) tenacity, Honeywell International Inc.). In each web ply, the fibers were coated with PRINLIN® B7137HV binder (commercially available from Henkel Technologies, Düsseldorf, Germany), which is a water-based dispersion of styrene-isoprene-styrene resin block copolymer (KRATON® D1107), and the combined structure contained 15 weight percent PRINLIN® B7137HV binder (based on the total weight of the two-ply web).

[0059] To form a multi-ply composite web, a first 64-inch (162.5 cm) wide continuous nonwoven unidirectional (axial) web of unidirectional filament bundles was formed from a parallel array of fiber / filament bundles containing 380 of the 1332 denier fiber / filament bundles. The array was formed using a process such as that shown in Figure 1, in which each bundle was unwound from a spool mounted on a cradle and subsequently passed through a collimating comb / heddle to arrange the bundles in a parallel array. The parallel array of bundles was then passed through an aqueous adhesive bath, such as that shown in Figure 2, of PRINLIN® B7137HV dispersion, to coat the filament bundles (typically each individual filament forming the bundle) with adhesive. The coated filament bundle array was then passed through a spreader bar, such as that shown in Figure 2, to achieve some partial filament spreading. The coated, partially spread filaments were then passed through a nip to remove excess binder composition coating, after which the coated fiber array was applied to one side of a silicone release paper (shown as an option in FIG. 2) and dried (e.g., by heating the web to about 100°C to evaporate the water from the aqueous binder composition). The dried web was then cut into a plurality of 64-inch by 64-inch squares. These squares were then rotated so that the component filaments were oriented 90° relative to their original axial orientation when the web was produced (as shown in FIG. 4). These squares were placed side by side, coplanar, and taped together with no spaces between the placed plies, thereby forming a continuous crosswise web (a continuous web of 90°-oriented filaments).

[0060] A second 64-inch wide continuous nonwoven unidirectional (axial) web was then produced in the same manner as the first web, with the same 1332 denier, 36.6 g / d UHMW PE filament bundles / fibers formed into another arrangement and then coated with the same aqueous PRINLIN® B7137HV dispersion according to the coating method shown in Figure 2. This web was not cut but remained as an axial web, with its component filaments oriented in the same longitudinal direction, i.e., 0°, as the filament bundles were initially delivered from the clair and passed through the collimating comb. At this point, rather than combining the coated axial arrangement with silicone release paper as shown in Figure 2, it was combined with the previously produced transverse web, and the two webs were combined while the binder composition coating on the 0° axial web was still wet. During this process (priming by wet lamination), pressure of approximately 1 psi was applied to the webs as they passed through the combining rollers, and heat was applied to the combined web (to a temperature of approximately 100°C) to coat the aqueous binder composition. of water was allowed to evaporate, thereby forming a dry two-ply web with the filament directions of the plies orthogonally oriented at 0° / 90°.

[0061] A second two-ply web was then formed, manufactured exactly as described immediately above. The two two-ply webs were then coextensively adjacent (placed one above the other with complete coverage on all sides), maintaining a 0° / 90° / 0° / 90° orientation of the filament webs. The two two-ply webs were then lightly adjacent, one on each outer surface of the combined web, to continuous films / sheets of continuous low-density polyethylene (LLDPE), each film having a thickness of approximately 0.35 mil (8.9 μm).

[0062] The stack of two continuous 2-ply webs, along with the two outer LLDPE films on the webs, was then transferred to a flat laminator and consolidated under heat and pressure by wet lamination to create a 4-ply composite web. The lamination temperature was 110°C with a continuous pressure of approximately 200 pounds per square inch applied to the adjacent webs. The resulting structure was a one-piece, monolithic, 4-fibrous ply composite web comprising (film / 0° web / 90° web / 0° web / 90° web / film). Multiple square / rectangular test specimens were then cut from this 4-ply composite web and tested for their tensile, shear, and friction properties.

[0063] Comparative Examples 1 to 5 Tensile property test Tensile properties were obtained according to ASTM D3019 and ASTM D5034 / 5035 guidelines on a standard Instron 1332 load frame machine using MTS647.10A hydraulic grips. Test specimens were attached directly to the grips, and tension was applied by pulling the specimen away from the grips until failure. All tests were performed at room temperature (72°F) at a tensile loading rate of 0.1 in / min. Displacement and strain data were obtained using a DIC (digital image correlation) instrument to capture digital images, which were analyzed using VIC3D software from Correlated Systems Inc. Tensile specimens were tested to failure.

[0064] [Table 1] Table 1

[0065] Comparative Examples 6 to 9 Shear property test Shear properties were obtained using the ASTM D8067 test method. Rather than mounting the test specimen directly in hydraulic grips as in Comparative Examples 1-5, the test specimen was mounted on a rectangular picture frame-type fixture held in a standard Instron 1332 load frame machine. The same MTS647.10A hydraulic grips were then used to grip the rectangular frame as in Comparative Examples 1-5. The shear properties of the material are tested under quasi-static, very small loads (shear strains) with the load applied at two diagonal corners according to the ASTM D8067 method. The shear strain rate for the test was 0.1 in / min, and the specimens were then subjected to a 100% shear strain. The load was applied until buckling. Displacement and strain data were acquired using a DIC instrument and analyzed using VIC3D software from Correlated Systems Inc., as described above for Comparative Examples 1-5.

[0066] [Table 2] Table 2

[0067] Comparative Examples 10 to 13 Friction Coefficient Test The samples were also tested to determine their coefficient of friction, with friction properties measured according to the ASTM G115 method. Friction tests were performed on a GCTS biaxial test frame (International Test Equipment, Tempe, Arizona) with a shear actuator moving the sample and a normal actuator applying a normal force to the sample at a constant displacement. Friction data was obtained from the system using two load cells: a shear actuator and a shear actuator.

[0068] [Table 3] Table 3

[0069] Examples 14 to 26 A novel continuous four-ply fibrous composite web (film / 0° web / 90° web / 0° web / 90° web / film) was produced from the same materials (same fibers, binder, and film type) using two two-ply 0° / 90° continuous webs in exactly the same manner as described above in Comparative Examples 1-13. However, rather than consolidating the two two-ply continuous webs and the LLDPE film together at high pressure in a flatbed laminator, the two two-ply webs and film were consolidated in an equal-pressure steel belt press (double steel belt press) as shown in Figure 3. In this regard, the two two-ply fibrous webs and the two outer LLDPE films were abutted and then fed together into the equal-pressure steel belt press at once, and the combination was passed through the press at a speed of 4 meters per minute. The press had a heating zone set to 110°C and was set to apply a pressure of 80 bar (1160 psi) to the laminated web and film passing through the press heating zone, subjecting it to an axial extensional stress while simultaneously heating it. After passing through the heating zone, the material (i.e., the combined web and film) was then cooled to below 50°C in a cooling zone of the press set to a temperature of approximately 40°C while maintaining the same 80 bar pressure on the material as in the heating zone. The material was then cooled at room temperature (approximately 72°C) while still under the same pressure. The resulting structure was a one-piece, monolithic, four-ply fibrous composite web comprising (film / 0° web / 90° web / 0° web / 90° web / film). Multiple square / rectangular test specimens were then cut from this four-ply composite web and tested for their tensile, shear, and friction properties.

[0070] Examples 14 to 18 Tensile property test Tensile properties were obtained according to ASTM D3019 and ASTM D5034 / 5035 guidelines on a standard Instron 1332 load frame machine using MTS647.10A hydraulic grips. The test specimen was attached directly to the grips, and tension was applied by pulling the specimen away from the grips until failure. All tests were performed at room temperature (72°F) at a tensile loading rate of 0.1 in / min. Displacement and strain data were obtained using a DIC (digital image correlation) instrument to capture digital images, which were analyzed using VIC3D software from Correlated Systems Inc. Tensile specimens were tested to failure.

[0071] [Table 4] Table 4

[0072] Examples 19 to 22 Shear property test Shear properties were obtained using the ASTM D8067 test method. Rather than directly mounting the test specimens in hydraulic grips as in Comparative Examples 1-5, the test specimens were mounted on a rectangular picture-frame-type fixture held in a standard Instron 1332 load frame machine. The rectangular frame was then gripped using the same MTS647.10A hydraulic grips as in Comparative Examples 1-5. The shear properties of the materials are tested under quasi-static, very small loads (shear strains) with the load applied at two diagonal corners according to the ASTM D8067 method. The shear strain rate for the test was 0.1 in / min, and the specimens were loaded until they buckled. Displacement and strain data were obtained using DIC equipment and analyzed using VIC3D software from Correlated Systems Inc., as described above for Comparative Examples 1-5.

[0073] [Table 5] Table 5

[0074] Examples 23 to 26 Friction Coefficient Test The samples were also tested to determine their coefficient of friction, with friction properties measured according to the ASTM G115 method. Friction tests were performed on a GCTS biaxial test frame (International Test Equipment, Tempe, Arizona) with a shear actuator moving the sample and a normal actuator applying a normal force to the sample at a constant displacement. Friction data was obtained from the system using two load cells: a shear actuator and a shear actuator.

[0075] [Table 6] Table 6

[0076] Conclusion: The fibers in each fiber ply were stretched axially as a result of the axial extensional stress applied to the fibers in the press. The tenacity of each fiber in each ply increased by at least 1%, and the elongation to break of each fiber decreased by at least 1%, with the reduced elongation to break value ranging from about 2.0% to about 3.9%. Additionally, the new materials of the present disclosure exhibited higher tensile properties, lower elongation, greater shear strength, and a lower coefficient of friction compared to comparative materials not processed by the press. This specification includes the disclosure of the following inventions. [Item 1] 1. A fibrous article comprising one or more unidirectional fiber plies, each unidirectional fiber ply comprising a plurality of unidirectionally oriented fibers coated with a polymeric binder, the fibrous article having been primed and the fibers having been axially stretched in a compression device, the fibers having an initial elongation to break value before being axially stretched and a reduced elongation to break value after being axially stretched, the initial elongation to break value being greater than the reduced elongation to break value, and the reduced elongation to break value being between about 2.0% and about 3.9%. [Item 2] Item 2. The fibrous article according to item 1, wherein the initial elongation at break value is 3.0% or more. [Item 3] Item 2. The fibrous article according to item 1, wherein the initial elongation at break value is 4.0% or more. [Item 4] 1. A method for improving fiber tensile properties of a fibrous prepreg composite, the method comprising: a) providing at least one fibrous prepreg composite including one or more unidirectional fiber plies, each of said plies including a plurality of unidirectionally oriented fibers coated with a polymeric binder, said fibers having a toughness of 7 g / denier or greater, a tensile modulus of 150 g / denier or greater, and an elongation to break of at least 2.0%, said fibers having a softening temperature and a melting temperature; b) heating the fibers of the composite to a temperature above the softening temperature of the fibers but below the melting temperature of the fibers; c) subjecting the composite to an axial extensional stress while the fibers are at the temperature above the softening temperature of the fibers but below the melting temperature of the fibers, wherein the fibers in each fiber ply are axially elongated by the axial extensional stress, thereby increasing the toughness of each fiber in each ply by at least 1% and decreasing the elongation to break of each fiber by at least 1%. [Item 5] 5. The method of claim 4, wherein the composite is subjected to the axial extensional stress while passing through a compression conveyor, and the axial extensional stress is at least about 0.25 kilogram-force (2.45 Newtons). [Item 6] 6. The method of claim 5, wherein the compression conveyor includes two pressure application elements, and the composite is compressed between the pressure application elements, whereby the pressure application elements simultaneously subject the composite to an axial tensile stress while simultaneously conveying the composite through the compression conveyor. [Item 7] 7. The method of claim 6, wherein the composite passes through the compression conveyor as a continuous web, the web being maintained under continuous and constant axial tension. [Item 8] Item 6. The method of item 5, wherein each composite comprises two cross-plied unidirectional fiber plies that are consolidated together, and the composite is subjected to multiaxial tensile stress while passing through a compression conveyor. [Item 9] Item 6. The method of item 5, wherein each composite comprises two cross-plied unidirectional fiber plies that are consolidated together, and the composite is subjected to multiaxial tensile stress while passing through a compression conveyor. [Item 10] 10. The method of claim 9, wherein a plurality of the composite materials are inserted into the compaction conveyor, and the composite materials are not attached to one another upon insertion into the compaction conveyor.

Claims

[Claim 1] 1. A fibrous article comprising one or more unidirectional fiber plies, each unidirectional fiber ply comprising a plurality of unidirectionally oriented fibers coated with a polymeric binder; the fibrous article being primed by pressing at a pressure of less than 0.0345 MPa (5 psi), the fibers being axially stretched in a compression device, the compression device being a compression conveyor including two pressure-applying elements; the fibers having an initial elongation to break value before being axially stretched and a reduced elongation to break value after being axially stretched, the initial elongation to break value being greater than the reduced elongation to break value, the reduced elongation to break value being between 2.0% and 3.9%, the elongation to break being measured by ASTM D2256.

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