BALLISTIC-RESISTANT CURVED MOLDED ARTICLE

Abstract

The present invention provides a process for producing a ballistic-resistant curved molded article said process comprising pressing in a mold a stack comprising a plurality of layers of unidirectionally aligned polyolefin fibers substantially absent a bonding matrix and a plurality of layers of adhesive; characterized in that a means for dispersing pressure is employed against at least one surface of the stack. Also provided are a ballistic-resistant curved molded article and a press-pad having substantially the shape of a curved mold.

Claims

1. A process for producing a ballistic-resistant curved molded article, said process comprising pressing in a mold a stack comprising: a plurality of layers of unidirectionally aligned polyolefin fibers substantially absent a bonding matrix; and a plurality of layers of adhesive, characterized in that a means for dispersing pressure is employed against at least one surface of the stack.

2. A process according to claim 1, wherein a means for dispersing pressure is employed against two surfaces of the stack.

3. A process according to claim 1, wherein the means for dispersing pressure is a press-pad.

4. A process according to claim 1, wherein the means for dispersing pressure is a fluid in any one of an autoclave, a hydroclave or a diaphragm molding machine.

5. A process according to claim 1, wherein the ballistic-resistant curved molded article comprises at least one filler ply.

6. A process according to claim 1, wherein the ballistic-resistant curved molded article is a helmet shell or a radome.

7. A process according to claim 1, wherein the total amount of adhesive present in the ballistic-resistant curved molded article is less than 15.0 wt. % based on the total weight of the molded article.

8. A process according to claim 1, wherein each layer of unidirectionally aligned polyolefin fibers is oriented at an angle of from 45 to 135 with respect to the orientation of an adjacent layer of unidirectionally aligned polyolefin fibers.

9. A process according to claim 1, wherein each layer of unidirectionally aligned polyolefin fibers is separated from an adjacent layer of unidirectionally aligned polyolefin fibers by a layer of adhesive.

10. A ballistic-resistant curved molded article obtainable by a process as defined in claim 1.

11. A ballistic-resistant curved molded article, which article comprises a plurality of layers of unidirectionally aligned polyolefin fibers which layers are substantially absent a bonding matrix; and a plurality of layers of adhesive, which ballistic-resistant curved molded article has an areal density of at most 11 Kgm.sup.2 and meets NIT level III.sup.+ performance against 7.6239 mm MSC (AK47).

12. A ballistic-resistant curved molded article according to claim 11, which ballistic-resistant curved molded article comprises at least one filler ply.

13. A ballistic-resistant curved molded article according to claim 11, which ballistic-resistant curved molded article is a helmet shell or a radome.

14. A ballistic-resistant curved molded article according to claim 11, wherein the total amount of adhesive present in the ballistic-resistant curved molded article is less than 15.0 wt. % based on the total weight of the molded article.

15. A press-pad having substantially the shape of a pressing surface of a curved mold.

Description

EXAMPLES

Reference Experiment 1a) and b)

[0079] 400 mm400 mm sheets of unidirectionally aligned fiber layers, available as HB210 from DSM Dyneema, Heerlen, Netherlands, were stacked to form an assembly having an areal density of 13.0 Kgm.sup.2. The sheets each comprised 4 layers, each layer comprising unidirectionally aligned fibers of UHMWPE embedded in a matrix of 17% of a polyurethane resin, and layered in the configuration of fiber direction 0/90/0/90. In total, 96 sheets were used, with the alternating 0/90 direction of adjacent layers maintained throughout the stack. The assembly of sheets was pressed at either a) 16.5 MPa or b) 31.7 MPa, in each case at 125 C. for 40 minutes followed by a cooling period of 20 min at 2 MPa. A molded article having an areal density of 13.0 Kgm.sup.2 resulted. The molded article was shot with a 7.6239 mm MSC (AK47) bullet in order to determine E.sub.abs.

Reference Experiment 2a) and b)

[0080] A precursor sheet was produced from 40 yarns of Dyneema SK76 1760 dtex yarn, available from DSM Dyneema, Heerlen, Netherlands. Yarn was unwound from bobbins on a tension controlled creel and passed through a reed. Subsequently the yarns were spread to form a gap-less bed of filaments with a width of 320 mm by feeding the yarns over a spreading unit. The spread yarns were then fed into a calender. The rolls of the calender had a diameter of 400 mm and the applied line pressure was 2000N/cm. The line operated at a line speed of 8 m/min and at a roll surface temperature of 154 C. In the calender the yarns were fused into a fibrous tape. The tape was removed from the calender by the first roller-stand. A powder scattering unit was placed between the calender and the first roller-stand applying 7 wt. % Queo 1007 powder, available form Borealis, Vienna, Austria to the upper surface of the tape. The tape with powder was calendered at elevated temperature and wound onto a roller stand.

[0081] A fibrous tape with a width of 320 mm and a thickness of 46 m was obtained. The fibrous tape had a tenacity of 35.4 cN/dTex and a modulus of 1387 cN/dTex.

[0082] Five of said tapes were aligned in parallel and abutting to form 1600 mm wide sheet. A second, identical, sheet of five tapes was formed on top of the first sheet, with the adhesive layers of both sheets facing upwards, but with the fibers of adjacent sheets aligned perpendicularly. A two-layered, cross-plied precursor sheet having an areal density of 95 gm.sup.2 resulted. This precursor sheet was cut into 400mm400 mm square precursor sheet. Multiple square precursor sheets were stacked, making sure the alternating 0/90 direction of the tape layers was maintained. The stack of precursor sheets was processed into a molded article of 9.8 Kgm.sup.2. The molded article contained 103 square precursor sheets (206 layers of unidirectional aligned tapes). The stack of precursor sheets was pressed into a molded article at either a) 16.5 MPa or b) 31.7 MPa, in each case at 145 C. for 40 minutes followed by a cooling period of 20 min at 2 MPa.

[0083] The molded article was shot with a 7.6239 mm MSC (AK47) bullet in order to determine E.sub.abs.

Comparative Experiment A

[0084] Reference Experiment 1a) was repeated, except that, before stacking the 4-layer sheets, in five of said 4-layer sheets, six squares of 5 cm5 cm were cut, evenly distributed over the area of the sheet with 2 edges parallel to the fiber directions. The cut squares in each of the five sheets were in the same position such that when stacked the cut squares were superimposed. Molded articles were pressed as described in Reference Experiment 1. The molded article was shot with a 7.6239 mm MSC (AK47) bullet in order to determine E.sub.abs. The article was shot each time at the center of the area of one of the cut squares. The obtained E.sub.abs expressed as a percentage of the E.sub.abs of Reference Experiment 1a) is given in Table 1.

Comparative Experiment B

[0085] Comparative Experiment A was repeated except that pressing was carried out at 31.7 MPa. Further, a press-pad was used on one side used during pressing. The press-pad was made of silicone having a Shore A hardness of 50+/5 and was 1.6 mm thick. E.sub.abs expressed as a percentage of the E.sub.abs of Reference Experiment 1b) is given in Table 1.

Comparative Experiment C

[0086] Reference Experiment 2a) was repeated, except that, before stacking the 2-layer precursor sheets, in six of said 2-layer precursor sheets, six squares of 5 cm5 cm were cut, evenly distributed over the area of the sheet with 2 edges parallel to the fiber directions. The cut squares in each of the six precursor sheets were in the same position such that when stacked the cut squares were superimposed. Panels were pressed as described in Comparative Experiment 2a). The molded article was shot with a 7.6239 mm MSC (AK47) bullet in order to determine E.sub.abs. The article was shot each time at the center of the area of the cut squares. The obtained E.sub.abs expressed as a percentage of the E.sub.abs of Reference Experiment 2a) is given in Table 1.

Comparative Experiment D

[0087] Comparative Experiment C was repeated except that pressing was carried out at 31.7 MPa. Further, E.sub.abs expressed as a percentage of the E.sub.abs of Reference Experiment 2b) is given in Table 1.

Example 1

[0088] Comparative Experiment D was repeated except that a press-pad was used on one side during pressing. The press-pad was made of silicone having a Shore A hardness of 50+/5 and was 1.6 mm thick. V.sub.50 expressed as a percentage of the V.sub.50 of Reference Experiment 2b) is given in Table 1.

TABLE-US-00001 E.sub.abs Areal Means for Comparison Performance Example density Pressure increasing with v Ref. Ex. No. [Kgm.sup.2] [MPa] homogeneity Ref. Ex. (AK47) [%] Comp. 13.0 16.5 None Ref. Ex. 1a) 58 Ex. A Comp. 13.0 31.7 2 mm Ref. Ex. 1b) 92 Ex. B silicone press-pad Comp. 9.8 16.5 None Ref. Ex. 2a) 25 Ex. C Comp. 9.8 31.7 None Ref. Ex. 2b) 41 Ex. D Ex. 1 9.8 31.7 2 mm Ref. Ex. 2b) 92 silicone press-pad

[0089] The results against 7.6239 mm MSC (AK47) threat show that significant reduction in E.sub.abs occurs when areal density is varied, shown by introducing cut squares into the stack of layers compared with material having uniform areal density. The material of Comparative Examples D and especially C show a more significant reduction than Comparative Example B. Use of the press-pad when pressing at 31.7 MPa (Example 1 and Comparative Example B) provides a E.sub.abs of the area of the cut squares almost as high as the corresponding material without squares cut (Reference Examples 1 b and 2b). Proportionally, this improvement is more pronounced for Example 1 (over Comparative Example C) than for Comparative Example B (over Comparative Example A).