WO2020127187A1

WO2020127187A1 - Ballistic-resistant molded article

Info

Publication number: WO2020127187A1
Application number: PCT/EP2019/085533
Authority: WO
Inventors: Jan Stolk; Raul Marcelino PEREZ GRATEROL; Bart SCHOONENBERG; Xiao Hu; Mohammad Faiz Bin MOHAMMAD-ZULKIFLI
Original assignee: Dsm Ip Assets B.V.
Priority date: 2018-12-21
Filing date: 2019-12-17
Publication date: 2020-06-25
Also published as: SG10201811534WA; TW202031473A

Abstract

The present invention relates to a ballistic-resistant molded article comprising a consolidated stack of layers, wherein said stack of layers comprises: - a plurality of core layers, said core layers comprising core fibers having a tenacity of at least 1.5 N/tex and a bonding matrix; and - at least one hybrid layer, wherein said hybrid layer comprises: i) polymeric fibers; ii) a polymeric resin; and iii) non-polymeric fibers; and to a process for producing such a ballistic resistant molded article.

Description

BALLISTIC-RESISTANT MOLDED ARTICLE

The present invention relates to a ballistic-resistant molded article comprising a consolidated stack of layers and a process for producing a ballistic resistant molded article. The article is preferably a ballistic resistant curved molded article, for example an insert for ballistic vests, a helmet shell or a radome.

Laminated composite materials are well-known for use as ballistic resistant molded articles. Reduction of weight, while maintaining ballistic and structural performance is an ongoing aim in the field. Typically, weight reduction is achieved by reducing the density of the composite and maintenance of ballistic performance is permitted by use of stronger fibers. An example is a switch from aramid-fiber based composites to ultrahigh molecular weight polyethylene (UHMWPE)-based composites. However, such density reduction has led to a reduction in structural performance, for example flexural rigidity or back face deformation (trauma). Accordingly, use of structurally rigid layers in combination with ballistic resistant layers has occurred.

WO2013/008178 describes a laminated composite for ballistic protection with a solid central thermoplastic polymer layer either sandwiched between two laminated thermoset material layers or comprising at least on one of the faces, a laminated thermoset material. A combination of UHMWPE plies sandwiched by carbon fiber plies is exemplified. The structure has low weight, good mechanical strength, improved ballistic properties and limited trauma.

Ballistic-resistant curved molded articles, for example helmet shells, comprising a consolidated stack of composite sheets comprising high tenacity fibers and a binder are also known in the prior art, for example from W02007/107359. A similar problem in reduction of areal density of the helmet shell occurs as for flat panels. A result is lower ear to ear rigidity of the helmet shell. Use of structurally rigid layers in helmet shell design is also known.

“The Development of a Hybrid Thermoplastic Ballistic Material With Application to Helmets” by Shawn M. Walsh, Brian R Scott and David M. Spagnuolo, Army Research Laboratory, ARL-TR-3700 (December 2005) describes a polyolefin helmet shell having an outer coating of a graphite-epoxy layer and a helmet shell having both an inner and outer coating of a graphite-nylon layer.

Carbon fiber-based composite layers are well known in combination with aramid composite layers in helmet designs; for example in WO2011163486A2, W02012097083A2, US8071008B. Carbon fiber composites are one example of composite materials comprising non-polymeric fibers. Other examples are glass fibers, basalt fibers, silicon carbide fibers or boron fibers. Typically, such fibers in a cured polymer matrix are well known in the art as being excellent structural materials. Glass fibers and carbon fibers are most commonly used. These materials are known to be light, strong, and stiff and therefore are increasingly applied in high performance structures. However, these materials have at least one disadvantage, namely that their impact resistance is very low or, in other words, their sensitivity to impact damage is very high.

Another problem with the use of non-polymeric fiber-based composite plies on the surfaces of a ballistic resistant composite article is the tendency for them to shatter when the article is struck by a blunt impact, e.g. a heavy low speed object, or a ballistic projectile, even if the projectile does not penetrate the article. This can cause formation of sharp edges and cause fragments to be ejected from the surface of the particle, risking injury and causing further damage. A particular problem occurs with a carbon fiber back-face of a ballistic-resistant molded article. Although the ballistic projectile may be prevented from penetrating, and may have relatively low back face deformation, the temporary deformation caused by impact of a ballistic projectile on the strike face of the molded article may cause shattering of the carbon fiber, leading to sharp edges on and/or fragments being ejected from the back face, typically towards the object or person being protected by the ballistic-resistant molded article.

In the case of a carbon fiber composite layer on the inside surface of helmet shell, sharp edges on the surface of the liner or fragments ejected from the carbon fiber layer may strike the wearer’s head causing serious injury.

WO2018/185047 describes a hybrid sheet comprising high- performance polyethylene fibers; a polymeric resin; and non-polymeric fibers. The sheet has improved flexural strength and bending strength, while maintaining high impact resistance properties against blunt impact. It is described in application in the automotive, aerospace, military, wind and renewable energy, marine and sports equipment fields (including sports helmets).

However, such a sheet is not known in combination with other materials. Nor is it known in the field of ballistic-resistance. Ballistic projectiles strike an article at higher velocity than objects causing blunt trauma. Ballistic projectiles are also more pointed and smaller and so more likely to penetrate an article. Accordingly, the forces acting on an article are quite different between a ballistic projectile and an object causing blunt trauma. An object of the present invention is to provide a ballistic-resistant molded article which avoids the shattering or fragmenting of surface layers compared with prior art articles. A further object is the provision of a ballistic-resistant molded article having reduced back face deformation. Further objects are the provision of a ballistic-resistant article having low weight, high ballistic resistance and high flexural rigidity. A still further objective is provision of a ballistic-resistant molded article, which is more receptive to coatings or adhesives.

The present inventors have developed a ballistic-resistant molded article with a surface layer which is surprisingly resistant to shattering on impact by a ballistic projectile. The ballistic-resistant molded article further has reduced back face deformation. Further, it shows increased flexural strength and stiffness and improved acceptance of adhesives and coatings.

Accordingly, the present invention provides ballistic-resistant molded article comprising a consolidated stack of layers, wherein said stack of layers comprises:

- a plurality of core layers, said core layers comprising core fibers having a tenacity of at least 1.5 N/tex and a bonding matrix; and

- at least one hybrid layer, wherein said hybrid layer comprises:

i) polymeric fibers;

ii) a polymeric resin; and

iii) non-polymeric fibers.

Further, the present invention provides a process for producing a ballistic-resistant molded article, said process comprising

a) providing a plurality of core layers, said core layers comprising fibers having a tenacity of at least 1.5 N/tex, and a bonding matrix; b) providing at least one hybrid layer, wherein said hybrid layer comprises: i) polymeric fibers;

ii) a polymeric resin;

iii) non-polymeric fibers;

c) arranging the core layers and hybrid layers into a stack of layers; and consolidating in a mold said stack of layers.

As used herein, a consolidated stack refers to multiple layers which have been pressed together to produce a single article. Consolidating means pressing together to produce a single article. Consolidation typically is done at elevated temperature. As used herein, a molded article refers to an article which has been pressed to produce an article of a specified shape. The shape may be flat or curved. Consolidation and molding may be achieved in the same action. As used herein a “curved” molded article is a non-planar molded article. It has a three-dimensional, rather than two-dimensional form. The article may have single or multiple curves. An example of a flat molded article is a panel. An example of a curved molded article having single curve is an insert or plate for a ballistic-resistant vest. An example of a curved molded article having multiple curves is a helmet shell or dome-shaped radome.

As used herein the term“a plurality” means an integer greater than 1. Melting point of the lowest melting component in the stack of layers takes into account the melting points of all components of all layers in the stack of layers. For example, the lowest melting point of all resins, matrices, bonding agents, films, fibers. Typically, fibers do not have such lowest melting points.

As used herein, average thickness is measured by taking at least 5 measurements distributed over the ballistic-resistant article, each measurement spaced apart from the other measurements by at least 5 cm, and calculating the mean value.

As used herein, areal density is calculated by multiplying the average thickness by the density of the ballistic-resistant molded article.

By fiber is herein understood an elongated body, the length dimension of which is much greater than the transverse dimensions of width and thickness. Accordingly, the term fiber includes filament, ribbon, strip, band, tape, and the like having regular or irregular cross-sections. The fiber may have continuous lengths, known in the art for example as filament or as continuous filament, or discontinuous lengths, known in the art as staple fibers. A yarn for the purpose of the invention is an elongated body containing many individual fibers. By individual fiber is herein understood the fiber as such. Preferably the fibers are tapes, filaments or staple fibers.

The core fiber or polymeric fiber may be a polymer chosen from the group consisting of polyamides and polyaramides, e.g. poly(p-phenylene

terephthalamide) (known as Kevlar®); poly(tetrafluoroethylene) (PTFE); poly{2,6- diimidazo-[4,5b-4’,5’e]pyridinylene-1 ,4(2,5-dihydroxy)phenylene} (known as M5);

poly(p-phenylene-2, 6-benzobisoxazole) (PBO) (known as Zylon®); liquid crystal polymers (LCP); poly(hexamethyleneadipamide) (known as nylon 6,6), poly(4- aminobutyric acid) (known as nylon 6); polyesters, e.g. poly(ethylene terephthalate), poly(butyleneterephthalate), and poly(1 ,4 cyclohexylidene dimethylene terephthalate); polyvinyl alcohols; and also polyolefins, for example homopolymers and copolymers of propylene and homopolymers and copolymers of polyethylene.

Preferably, core fibers have a tenacity of at least 1.8 N/tex, more preferably at least 2.0 N/tex, even more preferably at least 2.5 N/tex and most preferably at least 3.5 N/tex. Preferably, the core fibers are polyethylene fibers; more preferably high molecular weight (HMWPE) fibers; most preferably ultrahigh molecular weight polyethylene (UHMWPE) fibers.

Preferably, polymeric fibers of the hybrid layer have a tenacity of at least 1.5 N/tex, more preferably at least 1.8 N/tex, more preferably at least 2.0 N/tex, even more preferably at least 2.5 N/tex and most preferably at least 3.5 N/tex.

Preferably, the polymeric fibers are polyethylene fibers; more preferably high molecular weight (HMWPE) fibers or ultrahigh molecular weight polyethylene (UHMWPE) fibers; most preferably ultra-high molecular weight polyethylene.

The polymeric fibers may be different or identical to the core fibers. The polyethylene present in the core fibers or polymeric fibers may be linear or branched, whereby linear polyethylene is preferred. Linear polyethylene is herein understood to mean polyethylene with less than 1 side chain per 100 carbon atoms, and preferably with less than 1 side chain per 300 carbon atoms; a side chain or branch generally containing at least 10 carbon atoms. Side chains may suitably be measured by FTIR. The linear polyethylene may further contain up to 5 mol% of one or more other alkenes that are copolymerisable therewith, such as propene, 1 -butene, 1- pentene, 4-methylpentene, 1 -hexene and/or 1-octene.

The polyethylene is preferably of high molecular weight with an intrinsic viscosity (IV) of at least 2 dl/g; more preferably of at least 4 dl/g, most preferably of at least 8 dl/g. Such polyethylene with IV exceeding 4 dl/g are also referred to as ultra-high molecular weight polyethylene (UHMWPE). Intrinsic viscosity is a measure for molecular weight that can more easily be determined than actual molar mass parameters like number and weight average molecular weights (Mn and Mw).

The core fibers and polymeric fibers may be obtained by various processes, for example by a melt spinning process, a gel spinning process or a solid- state powder compaction process. Such processes are well known to the person skilled in the art.

Preferably, the polethylene fibers used in the invention are prepared by a gel spinning process. A suitable gel spinning process is described in for example GB-A-2042414, GB-A-2051667, EP 0205960 A and WO 01/73173 A1. In short, the gel spinning process comprises preparing a solution of a polyethylene of high intrinsic viscosity, extruding the solution into a solution-fiber at a temperature above the dissolving temperature, cooling down the solution-fiber below the gelling temperature, thereby at least partly gelling the polyethylene of the fiber, and drawing the fiber before, during and/or after at least partial removal of the solvent.

In the described methods to prepare polyethylene fibers drawing, preferably uniaxial drawing, of the produced fibers may be carried out by means known in the art. Such means comprise extrusion stretching and tensile stretching on suitable drawing units. To attain increased mechanical tensile strength and stiffness, drawing may be carried out in multiple steps.

As used herein the term“unidirectionally aligned” means fibers in a layer are orientated substantially parallel to one another, in the plane defined by the layer.

Preferably, a layer of unidirectionally aligned fibers is oriented at an angle of from 45° to 135° with respect to the orientation of an adjacent layer of unidirectionally aligned fibers. A preferred angle is 75° to 105°; for example about 90°. Optionally, each layer of unidirectionally aligned fibers is separated from an adjacent layer of unidirectionally aligned polyolefin fibers by a layer of adhesive.

The core layers used in the ballistic resistant molded article of the present invention comprise a bonding matrix. A bonding matrix essentially holds the fibers together. It may be applied between fibers in the same layer, such that it encloses the fibers in their entirety or in part, or between layers for example at the interface of two core layers.

In a preferred embodiment, the bonding matrix is a polymeric matrix material, and may be a thermosetting material or a thermoplastic material, or mixtures of the two. In the case the matrix material is a thermosetting polymer vinyl esters, unsaturated polyesters, epoxies or phenol resins are preferably selected as matrix material. In the case the matrix material is a thermoplastic polymer, polyurethanes, polyvinyls, polyacrylics, polyolefins or thermoplastic elastomeric block copolymers such as polyisopropene-polyethylene-butylene-polystyrene or polystyrene-polyisoprene- polystyrene block copolymers are preferably selected as matrix material. Preferably the bonding matrix consists of a thermoplastic polymer, which bonding matrix preferably completely coats the individual filaments of said fibers in a mono-layer. Preferably, the bonding matrix has a tensile modulus (determined in accordance with ASTM D638, at 25°C) of at least 75 MPa, more preferably at least 150 MPa and even more preferably at least 250 MPa, most preferably of at least 400 MPa. Preferably the bonding matrix has a tensile modulus of at most 1000 MPa.

Preferably, the bonding matrix is polyurethane or polyethylene. The bonding matrix may comprise further polymeric components as well as customary additives such as plasticizers, surfactants, fillers, stabilizers, colorants, etc.

In the case that the bonding matrix is a polyethylene, the polyethylene may be a polyethylene or ethylene copolymers, jointly or individually. It may comprise the various forms of polyethylene, ethylene-propylene copolymers, other ethylene copolymers with co-monomers such as 1 -butene, isobutylene, as well as copolymers of ethylene with hetero atom containing monomers such unsaturated carboxylic acids or derivatives thereof like acrylic acid, methacrylic acid, vinyl acetate, maleic anhydride, ethyl acrylate, methyl acrylate. In the absence of co-monomer in the polyethylene resin, a wide variety of polyethylene or polypropylene may be used amongst which are linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE), low density polyethylene (LDPE), or high density polyethylene (HDPE).

Typically the amount of bonding matrix present in the core layer is at least 2 wt%, preferably at least 5 wt%. In a preferred embodiment the amount of bonding matrix is at most 25 wt%, preferably at most 20 wt%, even more preferably at most 18 wt% and most preferably at most 16 wt%. In another preferred embodiment the amount of bonding matrix is between 2 and 25 wt%, preferably between 5 and 20 wt%, most preferably between 8 and 18 wt%, whereby the weight percentage are the weight of bonding matrix in the total weight of the core layers.

A hybrid layer comprises two different fibers: a polymeric fiber and a non-polymeric fiber.

By“non-polymeric fibers” is herein understood any fibers that do not contain a polymer. Alternative definition of non-polymeric fibers used in the present invention is fibers essentially not containing hydrogen atoms, which can be fibers that contain hydrogen atoms in an amount of less than 1 mass%, relative to the total mass of the fibers. Preferably, the non-polymeric fibers are selected from a group consisting of carbon fibers, glass fibers, wollastonite fibers, basalt fibers, silicon carbide fibers, boron fibers and mixtures thereof.

The non-polymeric fiber may have a titer of from 100 dtex to 100000 dtex, preferably of from 100 dtex to 50000 dtex. In particular, the carbon fibers or basalt or glass fibers may have a titer of between 500 and 40000 dtex, in particular between 650 and 32000 dtex and a filament count may be between 1000 and 48000. Mixtures of glass fibers, carbon fibers, wollastonite fibers and/or basalt fibers may also be used in any ratio according to the present invention. Preferably, the non-polymeric fibers used according to the present invention are fibers selected from a group consisting of carbon fibers, glass fibers, basalt fibers and/or mixtures thereof, more preferably the non-polymeric fibers used according to the present invention are fibers selected from a group consisting of carbon fibers and glass fibers.

The polymeric resin is typically a liquid (co)polymer resin impregnated in between the fibers and optionally subsequently hardened. Hardening or curing may be done by any means known in the art, e.g. a chemical reaction, or by solidifying from molten to solid state. Suitable examples include thermoplastic or thermoset resins. Preferably, the polymeric resin is an epoxy resin, a polyurethane resin, a vinylester resin, a phenolic resin, a polyester resin or a mixture thereof. The total concentration of the polymeric resin may be from 80 to 30 vol%, preferably from 70 to 40 vol%, yet preferably from 60 to 40 vol%, relative to the total volume of the hybrid sheet. Higher amount of polymeric resin adds disadvantageously to the total weight of the hybrid layers. Some voids may be present in the hybrid layers. Preferably, no voids are present in the hybrid layers. Any curing agent known in the art may be added to the matrix material, in any conventional amounts, by using any known method. The polymeric resin may further comprise at least one additive known in the art, in any conventional amounts, for example fillers, dyes, pigments, e.g. white pigment, flame- retardants, stabilizers, e.g. ultraviolet (UV) stabilizers, colorants. Such additives can be used to overcome common deficiencies of the fabric. The additives can be applied by any method already known in the art. The skilled person can readily select any suitable combination of additives and additive amounts without undue experimentation. The amount of additives depends on their type and function. Typically, their amounts are from 0 to 30 vol%, based on the total volume of the matrix material.

Preferably, the amount of polymeric resin is of from 0.5 to 25 vol%, preferably of from 1 to 20 vol%, most preferably of from 2 to 18 vol%, yet most preferably of from 2 to 10 vol%, related to the volume of the polymeric resin in the total volume of the hybrid sheet.

Preferably, the hybrid layer comprises from 15 to 50 vol% of the polymeric fiber, preferably at most 35 vol% polymeric fiber, and from 50 to 85 vol% non-polymeric fiber, relative to the total volume of the hybrid fabric. Higher amounts of the polymeric fiber result in lower values for mechanical properties. Lower amounts of the polymeric fiber result in lower impact strength properties and decrease of penetration resistance (i.e. out-of-plane impact resistance).

In one embodiment the polymeric resin is identical to the bonding matrix.

Preferably, the hybrid layer according to the present invention comprises:

i) from 5 to 35 vol.% of the polymeric fiber, relative to the total volume of the hybrid composite, with the polymeric fibers having a tensile modulus of at least 80 GPa, measured according to ASTM D885M-2014;

ii) from 20 to 60 vol% of the non-polymeric fiber, relative to the total volume of hybrid layer, and

iii) from 60 to 25 vol% of a polymeric resin, relative to the total volume of the hybrid layer.

The total sum of volumes of i) and ii) and iii) components and optionally, of volumes of conventional additives if present, should not exceed 100%.

A hybrid layer typically comprises a fabric of polymeric fibers and nonpolymeric fibers in a polymeric resin. A fabric can be of any type known in the art, for instance woven, non-woven, knitted, netted or braided and/or a technical fabric. These types of fabrics and way of making them are already known to the skilled person in the art. The areal density of fabrics is preferably between 10 and 2000 g/m², more preferably between 100 and 1000 g/m² or between 150 and 500 g/m². Suitable examples of woven fabrics include plain or tabby weaves, twill weaves, basket weaves, satin weaves, crow feet weaves, and triaxial weaves. Suitable examples of non-woven fabrics include unidirectional (UD) fibers, stitched fibers, veil and continuous strand mat.

A fabric is known in the art to be a three-dimensional (3D) object, wherein one dimension (the thickness) is much smaller than the two other dimensions. In a woven fabric, in general, the length direction is only limited by the length of the warp yarns whereas the width of a fabric is mainly limited by the count of individual warp yarns and the width of the weaving machine employed. The position of the warp yarns is defined according to their position across the thickness of the fabric, whereby the thickness is delimited by an outside and an inside surface.

The polymeric fibers may be used in weft and/or in warp directions in a woven fabric of the hybrid layer. Such construction shows better structural properties. Other constructions of the sheet may include non-polymeric fibers in warp direction and polymeric fibers only in weft direction or non-polymeric fibers and polymeric fibers in warp direction and polymeric fibers only in weft direction.

The hybrid layer can be made with any process known in the art. Suitable examples of known such processes include pre-impregnated fabrics process, hand lay-up, resin transfer molding or vacuum infusion process, autoclave process, press process.

The at least one hybrid layer may be present at one or both faces of the stack of layers; or it may be present within the stack of layers. Typically, the at least one hybrid layer is present at at least one face layer of the stack of layers. More preferably, it is present at the back face of the stack of layers, the back face being the face away from the ballistic threat. Alternatively, it is present at the strike face of the stack of layers, the strike face being the face towards the ballistic threat. More preferably, at least one hybrid layer is present at each face of the stack of layers.

The hybrid layer provides structural rigidity to the ballistic-resistant molded article. The higher proportion of hybrid layers to core layers, the higher the structural rigidity will be. However, typically the core layers have a ballistic resistance higher than the hybrid layers. Therefore, increasing the proportion of hybrid layers will lower the ballistic resistance. Accordingly, a balance needs to be found in the proportion of hybrid layers. Typically, the total areal density of the hybrid layers is from 0.5 to 40 wt.% of the total areal density of the ballistic-resistant molded article.

Preferably, the total areal density of the hybrid layers is from 1 to 30 wt.% of the total areal density of the ballistic-resistant molded article; more preferably, it is from 5 to 25 wt.%; or even 10 to 20 wt.%; most preferably about 15 wt.%.

Preferably, the ballistic-resistant molded article is a ballistic-resistant curved molded article. It may have a single curve, for example it may be an insert or plate for a ballistic-resistant vest. Alternatively, it may have multiple curves, for example it may be a helmet shell or dome-shaped radome. More preferably, the ballistic- resistant molded article is a helmet shell or a radome. Helmet shells as described herein enable the manufacture of ballistic resistant helmets which offer better protection than helmets known hitherto.

The ballistic-resistant molded article of the present invention may comprise further layers in the stack of layers than the core layers and the hybrid layers.

In particular, the ballistic-resistant curved molded article according to the present invention may comprise a so-called filler ply. Typically a helmet shell comprises a layer of, for example carbon fiber composite, which acts to increase stiffness and/or thickness. A layer of carbon fiber composite comprises woven or non- woven, including unidirectional oriented, carbon fibers and a polymeric resin.

Preferably, the ballistic-resistant curved molded article comprises one or more filler plies.

The inventors identified that the ballistic-resistant curved molded article according to the invention provides superior mechanical and anti-ballistic properties at reduced average thickness of the ballistic-resistant curved molded article. Therefore a preferred embodiment of the present invention concerns a ballistic- resistant curved molded article having an average thickness of at most 20.0 mm, preferably of at most 15.0 mm, more preferably of at most 10.0 mm and most preferably of at most 8.0 mm. Preferably the average thickness of the ballistic-resistant curved molded article of the present invention is between 4.0 and 15.0 mm, more preferably between 5.0 and 10.0 mm and most preferably between 5.5 and 8.0 mm. Ballistic-resistant curved molded articles and especially helmet shells in these preferred ranges show a compromise between ballistic-resistant performance and light weight of the molded article.

The ballistic-resistant molded article may further comprise a coating. For example, a coating imparting impact resistance ease of decoration or ease of release from the mold. The coating could be polyamide or polyethylene. Preferably the ballistic-resistant molded article of the present invention further comprises a coating comprising polyurea, for example, Paxcon^® available from Line-X^®, Huntsville,

Alabama, USA.

The hybrid layer of the ballistic-resistant molded article may provide enhanced adhesion compared with a core layer. A coating may adhere better to a hybrid layer than to core layers. Preferably, a coating is applied to a hybrid layer.

Further, fittings which are applied by adhesive, may adhere better to a hybrid layer than to a core layer. Accordingly, typically a fitting is applied to the hybrid layer. One example of fittings are pads, which are affixed to the inside surface of a helmet shell to provide a cushion against the head.

A helmet shell is the basic structure of a helmet, and provides the ballistic resistance. The shell excludes linings, straps, fittings, decoration and accessories. A preferred embodiment of the present invention is a ballistic-resistant helmet comprising the ballistic-resistant helmet shell.

The ballistic-resistant molded article may further comprise an additional material selected from ceramic, steel, aluminum, titanium, glass and graphite. The ballistic- resistant molded article and the additional material may be bonded or otherwise retained by support means, for example fiberglass, fabric, polymeric plastics and elastomers. The additional material is typically present at the strike face of the ballistic- resistant article.

The present inventors have found that ballistic resistant properties of a ballistic-resistant molded article according to the present invention may be improved, for example back face deformation and V50.

Back face deformation is effectively the size of the impact dent measurable on the non-impact side of the article. Typically, it is measured in mm of greatest deformation perpendicular to the plane of the impacted surface of the ballistic resistant article. It was surprisingly observed that the size of the impact dent is small. This is particularly important when the ballistic resistant molded article is used in armor, including helmets, since back face deformation may cause trauma on the human body, or skull and brain after being hit by a stopped projectile.

Preferably, a ballistic-resistant molded article according to the present invention has a back face deformation (BFD) of less than 20 mm when measured according to NIJ010601 / 9mm FMJ. Preferably the BFD is less than 18 mm, more preferably less than 16 mm.

V50 is a measure of the projectile-stopping performance of the ballistic-resistant molded article. It is the calculated velocity at which there is a 50% chance of a projectile penetrating the material. Preferably, a ballistic-resistant molded article according to the present invention has a V50 against 9mm FMJ 8g (DM41) of at least 400 ms^-1, at an areal density of the article of at least 3 kgrrr².

Flexural rigidity or stiffness is determined by resistance to bending of an article. In a ballistic-resistant helmet, ear-to-ear stiffness refers to the resistance to bending by pressing the helmet circumference at the location of the ears. This potentially reduces injury by compression of the helmet. A higher value is preferred.

In step a) of the process of the present invention, the core layers may be provided as a loose stack, i.e. unattached to one another. In another embodiment, the plurality of core layers in a) are provided in consolidated form, for example as a preform.

In the case that the ballistic-resistant molded article is a ballistic- resistant curved molded article, the mold in step d) is a curved mold. The stack is placed in an open mold, consisting of a female and a male part. Optionally the stack is clamped to one part of the mold, generally the female part. This clamping may be done through a so-called control member and is done in such a way that the stack is fixed in its position towards the said mold part, but that the stack can still slip and move during the closing of the mold, i.e. when moving of the male mold part into the female mold part. The clamping may be advantageously used when the curved shaped part has a substantial curvature, such as a helmet shell. Once the mold is closed e.g. by moving the male part into the female mold part and the stack is consolidated under

temperature and pressure into a curved molded article, such as a helmet shell.

Consolidation in step d) may also be carried out in an autoclave or a hydroclave. In that case only one mold part need be used. Typically, a female mold part is used. However, alternatively, a male mold part or a flat mold part may be used.

The process of the present invention may be applied in different orders. In one embodiment, the process is a two-step process. In such a process, the plurality of core layers is provided in step a) as a consolidated plurality of monolayers. The at least one hybrid layer is then applied in step c) to the consolidated plurality of core layers. The hybrid layer is typically applied in one of three ways, described below.

In one embodiment, the hybrid layer is provided in step b) as a fabric impregnated with a polymeric resin, a so-called“prepreg”, wherein said prepreg is applied in step c) to one or both faces of the consolidated core layers. The prepreg is then cured in the mold in step d) at the same time as being consolidated to the core layers. Step d) is typically carried out in an autoclave or hydroclave.

In an alternative embodiment, the hybrid layer is provided in step b) as a fabric, which is applied in step c) to at least one face of the consolidated core layers. A polymeric resin is then applied to the fabric. The hybrid layer is therefore provided in step b)“in-situ”, i.e. concurrently with step c), on the surface of the consolidated core layers. The hybrid layer is then cured in the mold in step d) at the same time as being consolidated to the core layers. Step d) is typically carried out in an autoclave or hydroclave.

In an alternative embodiment, the hybrid layer is cured independently of the consolidated core layers. The cured hybrid layer is typically rigid. The cured hybrid layer is then provided in step b) to at least one face of the

consolidated core layers (step c)); and is consolidated to the consolidated core layers in step d). In this embodiment, the cured hybrid layers and the consolidated core layers must have substantially the same shape. For example, in the case that the ballistic- resistant molded article is a ballistic-resistant curved molded article, both the plurality of core layers and the at least one hybrid layer may be provided already curved. In this embodiment, an adhesive may be applied between the consolidated core layers and the cured hybrid layers before they are consolidated in step d).

An advantage of a two-step process is that it permits different consolidation conditions for consolidation of the core layers; and for both curing and consolidation of the hybrid layers.

Typically, in step a’) consolidating is done at a pressure of from 8 to 30 MPa; and in step d) consolidating is done at a pressure of from 0.5 to 8 MPa. Preferably, consolidation is carried out in step a’) at a pressure of from 10 to 25 MPa, more preferably from 14 to 20 MPa. Preferably, consolidation is carried out in step d) at a pressure of from 1 to 6 MPa, more preferably from 2 to 4 MPa.

A second type of process is one-step consolidation process. Typically, in such a process the hybrid layer is provided as an impregnated fabric. An embodiment of such a process is to provide the plurality of core layers in step a) as an unconsolidated stack of layers. Core layers and hybrid layers are consolidated in step d). The impregnated fabric may be cured in step d) at the same time as the core layers are consolidated. In this embodiment, a hybrid layer may be provided within the stack of core layers, i.e. not necessarily at the face thereof.

An advantage of the one-step consolidation process is that it simplifies and shortens the production process. Further, it permits the hybrid layer to be located within the stack of core layers. It may also facilitate the forming of curved ballistic-resistant molded articles, as the core layers and hybrid layers are curved at the same time.

Typically, in the process of the present invention, the at least one hybrid layer is provided at at least one face of the stack of layers. In a preferred process, step c) comprises arranging at least one hybrid layer at each face of the stack of core layers.

Typically, the process of the present invention further comprises: e) cooling under pressure said consolidated stack to a temperature below the melting point of the lowest melting component in the stack of layers before releasing the ballistic-resistant molded article from the mold.

Typically, the process of the present invention further comprises, prior to step a), step a’) consolidating the plurality of core layers to be provided in step a).

Consolidation of the core layers, whether in step a’) or in step d) typically comprises heating to a temperature of from 80°C to 150°C, preferably from 90°C to 145°C and more preferably from 100°C to 140°C. In case of the bonding matrix being a thermoplastic resin the lower temperature is preferably higher than the melting temperature of the polymeric resin and the bonding matrix. In case of a thermosetting resin the lower temperature is governed by the temperature needed to initiate the chemical crosslinking process. The upper temperature should be below the melting temperature of the fibers. In the case of UHMWPE this is typically below 153°C. The pressure that is applied during the consolidation step of the core layers is typically between 1 and 40 MPa; preferably the pressure is between 5 and 35 MPa, more preferably between 8 and 30 MPa. In a preferred embodiment, the heating of the stack overlaps with the compression. In a further preferred embodiment, the compression of the stack is performed in more than one compression phases, whereby pressure is applied and released during the heating up phase of the stack. The maximum pressure applied to the stack during consolidation is also called the peak pressure. The consolidation time may be 5 minutes at the consolidation temperature and peak pressure provided here above, preferably consolidation time is at least 10 minutes, more preferably at least 15. The consolidated stack is preferably cooled in the mold, still under pressure. As soon as the consolidated stack has cooled to a temperature, typically of 80°C, preferably 50°C, the mold can be opened and the consolidated stack is released from the mold. Moreover, the consolidated stack may further be processed through known mechanical techniques as sawing, grinding, drilling to the desired final dimensions.

Test methods as referred to in the present application, are as follows:

• IV: the Intrinsic Viscosity is determined according to method ASTM D1601(2004) at 135°C in decalin, the dissolution time being 16 hours, with BHT (Butylated Hydroxy Toluene) as anti-oxidant in an amount of 2 g/l solution, by extrapolating the viscosity as measured at different concentrations to zero concentration.

• Tensile properties: tenacity and elongation at break (or eab) are defined and determined on monofilament fiber with a procedure in accordance with ISO 5079:1995, using a Textechno’s Favimat (tester no. 37074, from Textechno Herbert Stein GmbH & Co. KG, Monchengladbach, Germany) with a nominal gauge length of the fibre of 50 mm, a crosshead speed of 25 mm/min and clamps with standard jaw faces (4*4 mm) manufactured from Plexiglas® of type pneumatic grip. The filament was preloaded with 0.004 N/tex at the speed of 25 mm/min. For calculation of the tenacity the tensile forces measured are divided by the filament linear density (titer); values in GPa are calculated assuming a density of 0.97 g/cm³

• V50 ballistic limit was determined with a 9 mm FMJ DM41 threat shot at 400 x 400mm panels at different speeds to a maximum of 6 shots per panel. The final V50 was determined as the average of the three highest speeds with a stop, and the three lowest speeds yielding a perforation.

• Back Face Signature, also referred to as back face deformation, was measured according to NIJ010601 with a 9 mm FMJ DM41 threat shot at 427-445 m/s against a clay backing. The back face signature was defined as the remaining indentation of the clay.

• Flexural yield strength and flexural modulus. To study the flexural yield strength and flexural modulus of each panel concept, ASTM D790 standard for three-point bending test was adopted. Samples having span length, width, and thickness of 128mm, 13mm and plate height of approx. 4mm respectively were precut, using a band saw, from each pressed panel concept. Flexural modulus was calculated from the initial part of the force-displacement curve, as an apparent modulus for a homogeneous panel. The flexural yield strength was determined from the maximum force reached in the flexural test, as an apparent strength for a homogeneous panel.

· Removal of surface area of back face composite layer. The surface area removed from the composite layer on the back face after back face deformation shooting was measured from images of the back side of the panels using digital image processing software. The value was calculated as the average area of UHMWPE core exposed for 6 shots.

Examples 1 and 2

Preparation of UHMWPE panel

400 mm x 400 mm sheets of unidirectionally aligned fiber layers, available as HB26 from DSM Dyneema, Heerlen, Netherlands, were stacked to form an assembly. The sheets each comprised 4 layers, each layer comprising unidirectionally aligned fibers of UHMWPE embedded in a matrix of a polyurethane resin, and layered in the configuration of fiber direction 0 90 0 90°. In total, 12 sheets were used, with the alternating 0790° direction of adjacent layers maintained throughout the stack. The assembly of sheets was pressed at 16.5 MPa and 120°C for 40 minutes followed by a cooling period of 20 min to form a panel having an areal density of 3.1 kgrrr². Preparation of carbon fiber prepreg sheet

A sheet of 400 mm x 400 mm woven fabric of Torayca T300 3K carbon fibers in a 2/2 twill weave; said fabric having a Fabric Areal Weight (FAW) of 245 g/m² was impregnated with 156 g/m² of a two-part epoxy resin matrix, EP62-1 , supplied by Master Bond Inc. to form a prepreg layer. Parts A and B of the epoxy resin had densities of 1.17 g/cm³ and 0.98 g/cm³ respectively.

Preparation of hybrid UHMWPE-carbon prepreg sheet

A sheet of 400 mm x 400 mm woven fabric of Dyneema® SK75 fibers and Torayca T300 3K carbon fibers, available as DDCF002, in a 2/2 twill weave with a ratio of PE to carbon of 1 :2 yarn ratio in each the weft and the warp direction; a yarn number of 1760 dtex (SK75) and 2000 dtex (3K carbon); said fabric having a Fabric Areal Weight (FAW) of 235 g/m² was impregnated with 192 g/m² of two-part epoxy resin matrix, EP62-1.

Preparation of molded article

A number of prepreg sheets of either carbon fiber or of hybrid PE carbon-fiber, as defined in Table 1 , were laid on the UHMWPE panel. The combined panel and prepreg were placed in a flat mold, and were pressed in a hydraulic press at 2 MPa at 80°C for 30 minutes to form a molded article. The structures are defined in Table 1.

Table 1 :

The molded article was shot with a 9mm Full-Metal Jacket (FMJ) (DM41) projectile of 8-gram mass against clay backing, according to NIJ 0101.06. Tests were conducted with the carbon fiber layers or hybrid UHMWPE-carbon layers at the strike face and at the back face of the article being shot. V50 and Back Face Deformation (BFD) data were obtained from this test. In addition, where the carbon fiber layers or hybrid UHMWPE-carbon layers were at the back face the degree of carbon fiber removal was observed. Results are given in Table 2. Further, the flexural yield strength and flexural modulus were determined from samples cut from the articles produced, as defined above. These were determined both with carbon fiber layers or hybrid UHMWPE-carbon layers at the upper face and at the lower face. Results are listed in Table 2. Table 2:

These results demonstrate the reduction in shattering of using a hybrid UHMWPE-carbon layer compared with a carbon fiber layer at the back face of an article.

In addition, it was observed that the edges of the portion of removed surface of C. Ex 3 were sharp geometrical shapes, whereas the edges of the portion of removed surface of Ex. 2 had the appearance of frayed or displaced fibers.

The results also show that using hybrid carbon-UHMWPE layers in place of carbon fiber layer leads to no significant reduction in V50, BFD or flexural modulus or flexural rigidity. All parameters are increased compared to a panel having only a UHMWPE core.

The molded articles where also subjected to a blunt impact test by dropping a dart with a mass of 23.49 kg from a height of 0.3 m in the center of a 100 x 100 mm² panel sample. The clamp ring diameter was 40 mm and the spherical dart had diameters of 10 mm and 20 mm. The penetration depths of the darts into the panels was recorded and are shown in Table 3.

Table 3:

These results demonstrate the reduction of back face deformation under blunt impact when using a hybrid UHMWPE-carbon layer compared with a carbon fiber layer or no additional layer, either at the front or face of an article.

In addition, it was observed that after the test, the carbon layer of C. Ex. 1 and C. Ex. 2 showed substantial damage, portions of detached layers as well as sharp edges protruding from the back face of C. Ex. 3. Example 3

Preparation of UHMWPE panel

200 m x 200 mm sheets of unidirectionally aligned fiber layers, available as HB311 from DSM Dyneema, Heerlen, Netherlands, were stacked to form an assembly. The sheets each comprised 4 layers, each layer comprising

unidirectionally aligned fibers of UHMWPE embedded in a matrix of an ethylene copolymer resin, and layered in the configuration of fiber direction 0 90 0 90°. In total, 40 to 54 sheets were used, with the alternating 0790° direction of adjacent layers maintained throughout the stack. The assembly of sheets was pressed at 2 MPa for 5 minutes followed by 16.5 MPa for 65 minutes; wherein 40 minutes were at 138°C followed by a cooling period of 30 min to form a panel having an areal density of 5.1 to 6.8 kgrrr².

Preparation of hybrid UHMWPE-carbon prepreg sheets

Sheets of 200 mm x 200 mm woven fabric of Torayca T800 6K carbon fibers and Dyneema SK99 880dtex fibers in a 2/2 twill weave, with a PE fraction (SK99) of 18 weight%; said fabric having a Fabric Areal Weight (FAW) of 204g/m² were impregnated with 146g/m² of an epoxy resin matrix, MTC510, supplied by SHD Composites to form a prepreg layer having an Areal Weight of 350g/m².

Preparation of molded article

One or more prepreg layers were laid on the UHMWPE panel and the panel and prepreg were placed in a flat mold. The prepreg layers and UHMWPE panel were pressed in an autoclave at a pressure of 0.6 MPa and temperature of 110°C for 2 hours 25 minutes, excluding heating and cooling time, to form a molded article. Three samples were produced, wherein the weight ratio of UHMWPE layers to hybrid layers varied as 0:0, 95:5 and 75:25, respectively, all molded articles having an areal density of 6.8 kgrrr¹.

For each sample, 4 panels of 200 mm x 200 mm were shot, once each. The molded article was shot with a 9mm Full-Metal Jacket (FMJ) (DM41) projectile of 8-gram mass against clay backing, according to NIJ 0101.06. A steel frame was used to hold the sample. Projectile velocity was 430 ms^-1. Back Face Deformation (BFD) data were obtained from this test. Results are listed in Table 3, below.

Table 3:

These results show that the back face deformation is reduced as further hybrid layers are added to the core layers in the ballistic resistant molded article, from 0 wt.% to 25wt.%.

Claims

1. A ballistic-resistant molded article comprising a consolidated stack of layers, wherein said stack of layers comprises:

- at least one hybrid layer, wherein said hybrid layer comprises:

i) polymeric fibers;

ii) a polymeric resin; and

iii) non-polymeric fibers.

2. A ballistic-resistant molded article according to claim 1 , wherein the at least one hybrid layer is present at at least one face of the stack of layers.

3. A ballistic-resistant molded article according to claim 1 or claim 2, wherein the polymeric fibers of the hybrid layer are high molecular weight polyethylene or ultra-high molecular weight polyethylene.

4. A ballistic-resistant molded article according to any one of claims 1 to 3

wherein the non-polymeric fibers are selected from a group consisting of carbon fibers, glass fibers, wollastonite fibers, basalt fibers, silicon carbide fibers, boron fibers and mixtures thereof.

5. A ballistic-resistant molded article according to any one of claims 1 to 4,

wherein the core fibers are ultra-high molecular weight polyethylene.

6. A ballistic-resistant molded article according to any one of claims 1 to 5,

wherein the polymeric resin is an epoxy resin, a polyurethane resin, a vinylester resin, a phenolic resin, a polyester resin or a mixture thereof.

7. A ballistic-resistant molded article according to any one of claim 1 to 6, wherein the total areal density of the hybrid layers is from 1 to 30 wt.% of the total areal density of the ballistic-resistant molded article.

8. A ballistic-resistant molded article according to any one of claims 1 to 7, which is a ballistic-resistant curved molded article.

9. A ballistic-resistant curved molded article according to claim 8, which is a helmet shell or a radome.

10. A ballistic-resistant helmet comprising the ballistic-resistant helmet shell as defined in claim 9.

11. A process for producing a ballistic-resistant molded article as defined in any one of claims 1 to 9, said process comprising

ii) a polymeric resin;

iii) non-polymeric fibers;

c) arranging the core layers and hybrid layers into a stack of layers; and d) consolidating in a mold said stack of layers.

12. A process according to claim 11 , wherein the at least one hybrid layer is

provided at at least one face of the stack of layers.

13. A process according to claim 11 or claim 12, wherein the mold in step d) is a curved mold.

14. A process according to any one of claims 11 to 13, further comprising, prior to step a), step a’) consolidating the plurality of core layers to be provided in step a).

15. A process according to claim 14, wherein in step a’) consolidating is done at a pressure of from 8 to 30 MPa; and in step d) consolidating is done at a pressure of from 0.5 to 8 MPa.