BALLISTIC-RESISTANT ARTICLE BASED ON SHEETS WITH DISCONTINUOUS FILM SPLITS

Abstract

A ballistic-resistant articles, and methods for their preparation, based on sheets of UHMWPE films with discontinuous film splits, which combine flexibility and good ballistic properties, making them suitable for both soft-ballistic and hard-ballistic applications.

Claims

1. A ballistic-resistant article comprising a stack of sheets, the sheets comprising at least a first layer of unidirectionally oriented UHMWPE films and a second layer of unidirectionally oriented UHMWPE films, the direction of the films in the first layer being at an angle with respect to the direction of the films in the second layer, wherein the sheets comprise discontinuous film splits through at least the first and the second layers of films, the density of the film splits being from 1000 to 500000 film splits per m.sup.2 and wherein the sheets in the stack are consolidated, wherein at least 50% of the split centres of a first layer are aligned along a line essentially perpendicular to the surface of the layer with the split centres of an adjacent second layer.

2. The ballistic-resistant article of claim 1, wherein the film splits are separated by a radial distance, defined as the distance between a split centre and a neighbouring split centre in any direction of the film layer surface, from 0.5 to 100 mm.

3. The ballistic-resistant article of claim 2, wherein the density of the discontinuous film splits is from 5000 to 200000 film splits per m.sup.2.

4. The ballistic-resistant article of claim 1, wherein the split centres of the film splits are distributed forming straight lines the straight lines optionally being at an angle with respect to the length direction of the UHMWPE films.

5. The ballistic-resistant article of claim 1, comprising a thread stitched through at least part of the discontinuous film splits.

6. The ballistic-resistant article of claim 5 wherein the thread has a linear density of 10 to 500 dtex.

7. The ballistic-resistant article of claim 1, wherein the angle of the direction of the UHMWPE films in the first layer with respect to the direction of the films in the second layer is from 45 to 135 degrees.

8. The ballistic-resistant article of claim 1, wherein the sheets comprise 2 layers of UHMWPE films.

9. The ballistic-resistant article of claim 2, wherein an organic matrix material is present at least between the first and the second layers of UHMWPE films, wherein the organic matrix material is present in an amount of 0.1 to 10 wt. % based on the total weight of organic matrix material and UHMWPE films.

10. The ballistic-resistant article of claim 9, wherein the organic matrix material is a high density polyethylene (HDPE) or a low density polyethylene (LDPE).

11. The ballistic-resistant article of claim 1, wherein the stack of sheets has the sheets stitched together on their peripheral edges and/or the stack of sheets is placed inside a holding bag and/or the stack of sheets is shaped.

12. A process for the manufacture of a ballistic resistant article comprising a stack of sheets as defined in claim 1, the process comprising the steps of: a. providing a first layer of unidirectionally oriented UHMWPE films; b. providing a second layer of unidirectionally oriented UHMWPE films on top of the first layer of UHMWPE films to form a sheet comprising at least the first and second layers of unidirectionally oriented UHMWPE films, with the direction of the films in the first layer at an angle with respect to the direction of the films in the second layer; c. optionally applying an organic matrix material to the UHMWPE films prior to, after and/or during step a) and/or step b), wherein, if used, the organic matrix material is present at least between the first and the second layers of films; d. inducing discontinuous film splits through at least the first and second layers of UHMWPE films to form a sheet comprising discontinuous film splits with a film split density of 1000 to 500000 film splits per m.sup.2; e. stacking a plurality of sheets comprising discontinuous film splits induced according to step d) to form a stack of sheets f. consolidating the sheets prior to and/or after stacking according to step e) by applying pressure and optionally heat.

13. The process of claim 12 wherein inducing discontinuous film splits of step d) is performed by a needle to form the sheet with discontinuous film splits, optionally by a threaded needle whereby the sheet is provided with a thread stitched through at least part of discontinuous film splits.

14. The process of claim 12 further comprising stitching the stack of sheets together on the peripheral edges and/or placing the stack of sheets in a holding bag and/or shaping the stack of sheets by moulding, wherein shaping the stack of sheets by moulding is performed simultaneously to consolidating the sheets.

15. A ballistic resistant article obtainable by the process of claim 12.

Description

EXAMPLES

Example 1—Preparation of Sheets with Film Splits

Example 1A—Sheet Assembly of Two UHMPE Layers with HDPE Matrix and PES Thread Through the Splits

[0122] An UHMWPE film with a co-stretched HDPE matrix content of 1.Math.5 wt % with a thickness of 47 μm, a width of 132.8 mm and a modulus of 186.4 N/tex was used as a starting material.

[0123] A first layer of films was positioned on a moving belt under an angle of 45 degree with the running direction of the belt. A second layer of films was positioned on top of the first layer under an angle of 90 degrees with respect to the first layer.

[0124] The assembly of two film layers was transported to a sewing station. The layers were stitched together with a 48 dtex polyester (PES) sewing thread. Stitching lines ran parallel to direction of the moving belt. The stitching lines were separated by 0.2 inch (0.51 cm). The stitch length distance was 2.6 mm. The stitching resulted in the formation of film splits centred around the point where the needle impacted the film layers. After the stitching station, the sheet was wound on a core.

Example 1B—Sheet Assembly of Two UHMWPE Layers with HDPE Matrix and PES Thread Through Part of the Splits

[0125] A similar sheet was prepared as in Example 1A, with the difference that in the sewing station only 1 out of 5 equally spaced needles was equipped with PES sewing thread. This resulted in split lines separated by 0.2 inch (0.51 cm), i.e. having a film split distance perpendicular to the production direction of 0.2 inch (0.51 cm), but where only 1 out of 5 split lines had a thread defining a sewing line, i.e. defining a sewing thread-to-sewing thread distance of 1 inch (2.54 cm).

Example 1C—Sheet Assembly of Two UHMWPE Layers with HDPE Matrix and Copolyamide Fusible Thread Through the Splits

[0126] A similar sheet was prepared as in Example 1A, but where the sewing thread was replaced by a copolyamide fusible thread commercially available as Grilon K-85 75 dtex.

Example 1D—Sheet Assembly of Two UHMWPE Layers with LDPE Matrix and PES Thread Through the Splits

[0127] A similar sheet was prepared as in Example 1A, but where the matrix was changed from HDPE to LDPE and the matrix content was 2 wt %.

Example 2—Helmet from Sheets of UHMWPE Films with Discontinuous Film Splits

[0128] Sheets were prepared according to Example 1A, but with the difference that the stitch line distance was 0.4 inch (1.02 cm).

[0129] Each sheet was consolidated on a Schott and Meisner laminator at a temperature of 135° C. Two consolidated sheets were laminated together to form a 4-ply consolidated sheet. These 4-ply consolidated sheets were cut into a pattern consisting of a central circle and four lobes.

[0130] A total of 52 4-ply sheets cut as described above were stacked together, wherein each sheet was rotated over an angle of 3.9° compared to the previous sheet. In the middle the stack was fixed by hot welding at 90° C. The stack was put into a helmet shaped preform which was kept at a temperature of 60° C. and under a pressure of 4 bars for 4 minutes. Subsequently, the preform was put into a 60° C. preheated helmet mold and pressed at 55 bars. The mold was heated, keeping the pressure at 55 bars and after 30 minutes a temperature of 136° C. was reached. The temperature was held for further 30 minutes, and subsequently the mold was cooled down under a pressure of 55 bars to 60° C. within 30 minutes. Then the consolidated shape was removed from the mold. With a belt-saw the consolidated shape was cut into the final helmet shape.

[0131] The helmet was evaluated using 1.1 g fragment simulating projectiles (FSP). Results are shown in Table 1.

Comparative Example 1—Helmet from Sheets of UHMWPE Films without Discontinuous Film Splits

[0132] Using the same process of Example 2 a helmet was prepared based on commercially available Endumax XF33. Endumax XF33 is built-up of 4 UHMWPE film layers in a 0-0-90-90 configuration, where the two first layers are positioned in a brick construction (i.e. in the same direction but offset with respect to each other) and where the third and fourth layer are rotated 90° with respect to the first and second layer, said third and fourth layers being also positioned in a brick construction with respect to each other. All film layers are adhered to one another using a Kraton based glue.

[0133] A total of 52 sheets of Endumax XF33 were used to achieve a helmet of equal weight to that of Example 2.

[0134] The helmet was evaluated using 1.1 g fragment simulating projectiles (FSP). Results are shown in Table 1.

TABLE-US-00001 TABLE 1 Weight Trauma first shot v50 Sample (g) (mm) (m/s) Example 2 756 18 842 Comparative 755 21 750 Example 1

[0135] The results of Table 1 clearly show that the helmet shell prepared according to the invention (Example 2) has far better performance than helmets obtained with commercially available materials (Comparative Example 1). Furthermore, both in the preform step as in the final consolidation step, the material according to the invention (Example 2) was more easily drapable and formed more easily into the required shape resulting in a helmet shape with a more even thickness distribution.

Example 3—Hard Ballistic Ceramic Insert with Backing of UHMWPE Films with Discontinuous Film Splits

[0136] The UHMWPE sheet material obtained according to Example 1A was cut in sheets with dimensions 280×320 mm. 68 of these 280×320 mm sheets were stacked on top of a 8.5 mm Alotec-Ceramic insert. The total areal weight of the stack of 68 sheets (excluding the ceramic insert) was 5.1 kg/m.sup.2. One layer of commercially available Nolax foil F222031 of 250 g/m.sup.2, which serves as an adhesive, was placed in between the Alotec-Ceramic insert and the stack of sheets

[0137] The complete assembly was placed in a vacuum-bag and processed in a vacuum oven at 135° C. for 50 minutes. After the core temperature reached 135° C. the temperature was maintained for 10 minutes after which cooling was started until the core reached 60° C. Vacuum was maintained during the whole cycle.

[0138] During consolidation of the assembly the core temperature was measured with a thermocouple inserted in the middle of the stack.

[0139] It was found that the material according to the invention had good drapability enabling the production of high quality ceramic inserts with UMHPWE film based backing.

Comparative Example 2—Hard Ballistic Ceramic Insert with Backing of UHMWPE Films without Discontinuous Film Splits

[0140] The same procedure as in Example 3 was used to prepare an ceramic insert with a UHMWPE backing wherein instead of UHMPE sheets with film splits of Example 1A, sheets of commercially available Endumax XF33 (with the same configuration as described in comparative example 1) were used. 35 Endumax XF33 sheets were stacked on top of a 8.5 mm Alotec-Ceramic insert to form a UHMWPE backing having a total areal weight of 5.1 kg/m.sup.2 (excluding the ceramic insert). The complete assembly was placed in a vacuum-bag and processed in a vacuum oven at 140° C. After 46 minutes the core temperature reached 129° C. The temperature was maintained for 10 minutes after which cooling was started until the core reached 60° C. Vacuum was maintained during the whole cycle

[0141] The drapability of the UHMWPE backing was not as good as the drapability of the backing of Example 3 according to the invention. After consolidation the backing of Comparative Example 2 showed large wrinkles, which are undesired from a performance point of view and make it unsuitable for production of high quality ceramic inserts with UMHPWE film based backing.

Comparative Example 3—Soft Ballistic Panel with Aramid Strike Face and Backing of UHMWPE Films without Discontinuous Film Splits

[0142] An UHMWPE film with a co-stretched HDPE matrix content of 1.5 wt % with a thickness of 47 μm, a width of 132.8 mm and a modulus of 186.4 N/tex was used as a starting material.

[0143] A first 0-90 crossply of this material (sheet A) was produced on a Meyer lab laminator in the following manner:

[0144] Three rolls of said UHMWPE 133 mm wide film were positioned in an unwinding station. These films were led into the laminator with a minimal gap in between the films, so that the three films were aligned in parallel in abutting contact but without overlap, to form a bottom 0 degree film layer. On top of this 0 degree layer, three films of the same width and of 40 cm in length were positioned perpendicular to the 0 degree layer just before the entrance of the laminator forming a 90 degree film layer. The films in the 90 degree layer were manually positioned to achieve minimal overlap. After lamination a consolidated 0-90 cross-ply was obtained which was wound on a winding station.

[0145] In a second step, a second 0-90 cross-ply (sheet B) was produced on the same laminator as described above for sheet A except that, instead of three 133 mm wide films, four films were fed into the laminator, of which two had a width of 66.5 mm and two had a width of 133 mm.

[0146] In a third step, the cross-ply sheet A and the cross-ply sheet B were unwound and led into a laminator simultaneously to form and consolidate a 0-90-0-90 stack of cross-ply sheets. The consolidated stack of sheets was wound on a winding station.

[0147] The 0-90-0-90 consolidated cross-ply sheets were cut to dimensions of 30×30 cm and 24 of these 30×30 cm cuts were stacked on top of each other. This stack was combined with 6 layers of a Twaron CT619 fabric (a high tenacity aramid woven fabric) on the strike face and stitched completely around the edges to obtain a soft ballistic panel with an areal weight of 4.7 kg/m.sup.2.

[0148] In total two panels were prepared which were shot 4 times each with 0.44 Magnum. The back-face deformation was averaged over all 8 shots and found to be 45 mm.

Example 4—Soft Ballistic Panel with Aramid Strike Face and Backing of UHMWPE Films with Discontinuous Film Splits

[0149] Two sheet assemblies of two UHMPE layers with HDPE matrix and PES thread through the splits as described in Example 1A were fed into a laminator to obtain a consolidated material consisting of 4 film layers in a 0-90-0-90 configuration. 24 sheets of such 4-film layered material were cut with dimensions of 30×30 cm and stacked on top of each other. This stack was combined with 6 layers of a Twaron CT619 fabric on the strike face and stitched around completely to obtain a soft ballistic panel with an areal weight of 4.7 kg/m.sup.2.

[0150] In total two panels were prepared which were shot each 4 times with 0.44 Magnum. Average back-face deformation was 42 mm, clearly showing improved ballistic performance over a material without film splits as described in comparative example 3.

Evaluation of Stiffness

[0151] Stiffness of the different sheet material constructions was measured with a method derived from ASTM 4032.

[0152] Each sheet assembly of Examples 1A, 1B, and 1C was consolidated on a Schott and Meisner laminator at a temperature of 135° C. The sheet assembly of Example 1D was consolidated in a static press at 25 bar and 130° C.

[0153] As comparison, the stiffness was also evaluated for an Endumax XF33 sheet assembly (built-up of 4 UHMWPE film layers in a 0-0-90-90 configuration used in Comparative Examples 1 and 2) and for a sheet A assembly (built up of 2 UHMWPE film layers in a 0-90 configuration as described for sheet A in Comparative Example 3).

[0154] Samples of 10.2×20.4 cm were cut from each sheet material, with the 10.2 cm length in the direction of the stitch-lines (if present). Two samples were folded to obtain a four-sheet-layer sample of 10.2×10.2 cm. Several samples were placed on top of each other in the same way to form a stack. The stack was placed on a flat smooth polished metal plate with a circular hole of 1 inch diameter in the centre. The metal plate was positioned in a holder in a tensile tester, equipped with a rod positioned above the centre of the hole. In the stiffness measurement the rod pushed the stack through the hole with a speed of 5 mm/s. The stiffness was calculated as the initial slope in the 0 to 5 mm displacement region from the force-displacement curve. For comparison between samples the stiffness is divided by the areal weight resulting in a specific modulus (N/g).

[0155] The specific modulus of several materials is shown in Table 2. A lower specific modulus indicates an increased flexibility.

[0156] As can be seen from table 2 the stiffness is clearly diminished with materials comprising film splits according to the invention (Examples 1A-1D) when compared to materials not comprising film splits (Endumax XF33 and Sheet A).

TABLE-US-00002 TABLE 2 Sheet Specific modulus Sample Construction Film splits (N/g) Endumax XF33 0-0-90-90 no 157.5 Sheet A 0-90 no 88.7 Example 1A 0-90 yes 52.9 Example 1B 0-90 yes 54.4 Example 1C 0-90 yes 45.0 Example 1D 0-90 yes 70.0