Ballistic resistant sheets comprising multiple monolayers containing unidirectionally (UD) oriented reinforcing fibers with a matrix material are known, e.g., from U.S. Pat. Nos. 4,623,574, 5,766,725 and 7,527,854 and U.S. Patent Application Publication No. 2010/0064404 (the entire contents of each being expressly incorporated hereinto by reference).
A ballistic resistant sheet is furthermore known from WO2012/150169. In this publication a two-layer hybrid structure is disclosed comprised of a first layer (‘first stack’) comprising laminates with a first kind of yarn, and of a second layer (‘second stack’) comprising laminates with a second kind of yarn. The first kind of yarn and the second kin of yarn differ in linear density or thickness. Some matrix material are mentioned at page 3, including elastomer and epoxy resin, however no teaching on the use of these matrix materials are given. In the example the same matrix material, styrene-isoprene-styrene block copolymer, is used for all layers, comprising different types of aramid fibers. There is no mention of a 3 layer fiber based hybrid structure with tailored use of different matrix material per layer.
A ballistic resistant sheet is known from WO2008/077605. This publication discloses the manufacture of ballistic resistant sheets, whereby the ballistic resistant sheet is built up from monolayers with unidirectional polyethylene fibers and a matrix material. The matrix material disclosed in the example is a styrene-isoprene-styrene block copolymer. Furthermore a ballistic resistant moulded article is disclosed based on compressed ballistic resistant sheets combined with a ceramic strike face, optionally with a metal layer between the ceramic strike face and the ballistic resistant sheet. There is no mention of a 3 layer fiber based hybrid structure with tailored use of different matrix material per layer.
A ballistic resistant sheet is known from US2012/0244769. This publication discloses a method of producing a composite with a non-uniformly distributed matrix material. Example 1 discloses an aramid based unidirectional composite material with inhomogeneous distribution of a polyurethane-based matrix material, whereby a scrim material is bonded to the resin poor surface of the composite. This composite together with the same composite, however without the said scrim, are combined in a mould and compressed to form a shaped article. There is no mention of a 3 layer fiber based hybrid structure with tailored use of different matrix material per layer.
Preferably, each monolayer in the multi-monolayer sheet contains the UD oriented reinforcing fibers with the fiber direction in each monolayer being rotated with respect to the fiber direction in an adjacent monolayer. Such a ballistic resistant sheet is very suitable for use in compressed or moulded ballistic resistant articles such as panels and especially curved panels and articles (e.g., inserts, helmets, radomes). An alternative use of the ballistic resistant sheets of the embodiments disclosed herein, being a multi-monolayer construction including a core layer formed of at least one, preferably at least two, first monolayer comprised of first unidirectionally oriented reinforcing fibers and an elastomeric matrix material which is sandwiched between respective face layers, relates to soft ballistic articles, such as bullet-resistant vests.
There is continuous drive towards improved ballistic resistant articles, including moulded articles that enables the manufacture of compressed panels or ballistic resistant moulded articles with improved mouldability. Improved mouldability means that upon moulding of a ballistic resistant article, especially a curved ballistic resistant article, comprising several ballistic resistant sheets, a homogeneous product is obtained (i.e., a product having a visually identifiable homogeneity by a reduced or even absence of an inhomogeneous drape of the ballistic resistant sheets in the article after moulding). Additionally these sheets and articles should have a good, and preferably improved, ballistic resistance.
It is towards providing such ballistic resistant sheets and moulded articles therefrom that the present invention is directed.
In general, the embodiments disclosed herein relate to hybrid ballistic resistant sheets, articles which comprise such sheets and methods of making the same. According to some embodiments, the ballistic resistant sheets will include a core layer and face layers joined to respective opposing surfaces of the core layer. The core layer may include at least one, preferably at least two, first monolayer comprised of first unidirectionally (UD) oriented fibers and an elastomeric matrix material, while each of the face layers may include at least one, preferably at least two, second monolayer comprised of second UD oriented fibers and a non-elastomeric matrix material.
The first and second UD fibers may be the same or different from one another and may be selected from organic fibers and inorganic fibers. For example, at least one of the first and second UD fibers may be formed of inorganic fibers selected from the group consisting of glass fibers, carbon fibers and ceramic fibers. Alternatively or additionally, at least one of the first and second UD fibers may be formed of organic fibers selected from the group consisting of aromatic polyamide fibers, liquid crystalline polymer and ladder-like polymer fibers polyolefin fibers, polyvinyl alcohol fibers, and polyacrylonitriles fibers. According to some embodiments, at least one of the first and second UD fibers are formed of ultra high molecular weight (UHMW) polyethylene fibers, polybenzimidazole fibers, poly(1,4-phenylene-2,6-benzobisoxazole fibers and/or poly(2,6-diimidazo[4,5-b-4′,5′-e]pyridinylene-1,4-(2,5-dihydroxy)phenylene) fibers. In a particularly preferred embodiment the first and/or second UD fibers are formed of ultra high molecular weight (UHMW) polyethylene fibers. Preferably the UHMW polyethylene fibers are made from ultra high molecular weight polyethylene with an Intrinsic Viscosity of at least 4 dl/g, preferably of at least 6 dl/g, more preferably of at least 8 dl/g. The Intrinsic Viscosity is determined according to ASTM D1601 at 135° C. in decalin, the dissolution time being 16 hours, with DBPC as anti-oxidant in an amount of 2 g/l solution, by extrapolating the viscosity as measured at different concentrations to zero concentration.
The matrix materials of the core and face layers may comprise at most 20 mass % of the total mass of the monolayer(s).
The elastomeric matrix material employed in at least one of the first monolayers of the core will typically have a tensile modulus (i.e. secant modulus measured at about 23° C. according to ISO 527 at a strain of 100%) of less than about 3 MPa, sometimes less than about 2.5 MPa, for example less than about 2.0 MPa. This would lead to a further improved ballistic resistant sheet. According to some embodiments, the elastomeric matrix material may have a tensile modulus of less than about 1.5.
The elastomeric matrix may be comprised of at least one material selected from the group consisting of polybutadiene, polyisoprene, natural rubber, ethylene-propylene copolymers, ethylene-propylene-diene terpolymers, polysulfide polymers, polyurethane, polyurethane elastomers, modified polyolefins, chlorosulfonated polyethylene, polychloroprene, plasticized polyvinylchloride, butadiene acrylonitrile elastomers, poly(isobutylene-co-isoprene), polyacrylates, polyesters, polyethers, fluoroelastomers, silicone elastomers, thermoplastic elastomers, and ethylene copolymers. According to some embodiments, the elastomeric matrix material may comprise a block copolymer of a conjugated diene and a vinyl aromatic monomer. In this regard, the conjugated diene may be butadiene or isoprene while the vinyl aromatic monomer may be styrene, vinyl toluene or t-butyl styrene.
The non-elastomeric matrix material employed in at least one of the second monolayers of the face layers will typically have a tensile modulus (i.e. secant modulus measured at about 23° C. according to ISO 527 at a strain of 100%) of at least 3 MPa or greater, for example a tensile modulus of at least about 5 MPa or greater, e.g., up to about 500 MPa.
The non-elastomeric matrix material may be at least one selected from the group consisting of acrylates, polyurethanes, polyolefins—preferably polyethylene, modified polyolefins and ethylene vinyl acetate.
A ballistic resistant article may be provided which comprises consolidating the ballistic resistant sheet. In some embodiments, such a ballistic resistant article may exhibit a V50 of at least about 750 m/s (2470 fps) according to Stanag 2920 using a 7.62×39 mm mild steel core bullet.
The ballistic resistant sheets may be consolidated under an elevated pressure of at least about 16.5 MPa and an elevated temperature of preferably at least 10° C. below a temperature at which the fiber melts or at which mechanical properties of the first and second UD fibers deteriorates. Some embodiments will consolidate the sheets at an elevated pressure of at least about 20 MPa, for example at least about 25 MPa. The elevated pressure employed for sheet consolidation may be between about 16.5 MPa up to at least about 350 MPa, for example between 16.5 MPa to about 90 MPa, e.g., about 45 MPa.