Non-fibrous high modulus ultra high molecular weight polyethylene tape for ballistic applications
View Patent ↗A process for the production of a non-fibrous ultra high molecular weight polyethylene tape having a width of 1-inch or greater and a modulus of 1,400 grams per denier or greater. The non-fibrous UHMWPE tape is obtained by compression molding ultrahigh molecular weight polyethylene powder at a temperature below its melting point and then drawing and stretching the entire resultant compression molded UHMWPE sheet, with no slitting or splitting of the sheet, at a draw ratio of at least 100:1. The UHMWPE tape can be produced in weights of 6,000 to 90,000 denier or greater. The UHMWPE tape of the present invention minimizes the effect of stress concentrators that are prevalent with fibers and thereby enables the tape to be drawn at much higher draw ratios than is possible with fibrous UHMWPE. When used in ballistics panels, the high modulus high molecular weight polyethylene tape of the present invention improves ballistic performance by providing enhanced dissipation of the impact energy of a projectile.
1. A process for producing a non-fibrous ultra high molecular weight polyethylene tape comprising:
providing an ultra high molecular weight polyethylene (UHMWPE) powder having a viscosity-average molecular weight of at least 2,000,000;
compression molding said polyethylene powder at a temperature lower than the melting point of said polyethylene powder to form a friable polyethylene sheet;
calendering and shearing said friable polyethylene sheet between nip rolls to form a partially oriented polyethylene sheet;
drawing said partially oriented polyethylene sheet to form a polyethylene tape having a modulus of at least 1,400 grams per denier;
said drawing of said partially oriented polyethylene sheet includes a first stage drawing and a second stage drawing;
said first stage drawing includes a draw ratio where at the exit of said draw unit the total draw ratio of the compacted, calendered and partially oriented polyethylene sheet is between 14:1 and 24:1; and
said second stage drawing includes a draw ratio of at least 7:1 applied to said partially oriented polyethylene sheet.
2. The process of claim 1 that includes drawing to a total draw ratio of at least 100:1 wherein the draw ratio is defined as the length after stretching divided by the length before stretching.
3. The process of claim 1 wherein said polyethylene powder includes
a crystallinity of at least 76% as determined by X-ray diffraction; and
a heat of fusion of greater than 220 joules/gram as determined by differential scanning calorimetry.
4. The process of claim 1 wherein said non-fibrous polyethylene tape includes
a width of at least 1.0 inch;
a thickness of between 0.0015 and 0.004 inch; and
a denier of 6,000 or greater.
5. The process of claim 1 wherein said friable polyethylene sheet, said partially oriented polyethylene sheet, and said polyethylene tape are monolithic.
6. The process of claim 1 including
an efficiency defined by weight of first grade product produced divided by weight of polyethylene powder input; and
said efficiency is at least 95%.
7. The process of claim 1 wherein said temperature of said compression molding is at a temperature of 130° C. to 137° C.
8. The process of claim 1 wherein said compression molding includes a compression ratio of 2:1 to 4:1.
9. The process of claim 1 wherein said friable polyethylene sheet is a compacted sheet having a density of between 0.85 g/cm 3 and 0.96 g/cm 3 .
10. The process of claim 1 including preheating said friable polyethylene sheet to a temperature near the onset of melt prior to said calendaring and shearing.
11. The process of claim 1 wherein said UHMWPE tape is used in forming ballistic armor.
12. The process of claim 1 wherein said calendaring and drawing is at a tension of 0.5 to 5.0 grams per denier.
13. The process of claim 1 wherein said UHMWPE tape exits said drawing step at a speed of at least 20 meters per minute.
14. The process of claim 1 wherein said compression molding of said polyethylene powder includes a compression force of 25 kgf/cm 2 is applied to compress the UHMWPE powder to a compacted friable sheet.
15. The process of claim 1 wherein said polyethylene tape is substantially rectangular shaped and includes a width that is significantly larger than the thickness.
16. The process of claim 1 wherein said polyethylene tape includes
a thickness of between 0.0015 and 0.004 inch;
a width of between 1.0 and 8.1 inches; and
a width to thickness ratio of between 250:1 and 5400:1.
17. The process of claim 1 wherein said polyethylene tape includes a non-permeable surface.
18. The process of claim 1 wherein said polyethylene tape is free of slitting.