THREE DIMENSIONAL SHAPED ARTICLE

Abstract

The invention relates to a three dimensional shaped article having an outer and inner surface, the outer surface comprising at least one fabric (100) of polyethylene fibers having a tensile strength of at least 1.5 GPa, the fabric is impregnated with an acrylic based thermoplastic material. The three dimensional shaped article may further comprise monolayers with unidirectional aligned fibers. The three dimensionally shaped article has an improved surface appearance which would therefore need little or no post treatment and has good adhesion to coatings and paints.

Claims

1. Three dimensional shaped article having an outer and inner surface, the outer surface comprising at least one fabric (100) of polyethylene fibers having a tensile strength of at least 1.5 GPa, the fabric is impregnated with a an acrylic based thermoplastic material.

2. The article according to claim 1 wherein the acrylic based thermoplastic material has a glass transition temperature of at least 25 C.

3. The article according to claim 1 wherein the amount of acrylic based thermoplastic material is between 4 and 35 wt %.

4. The article according to claim 1, wherein the fibers are UHMWPE fibers.

5. The article according to claim 1, wherein the fabric is a plain woven fabric.

6. The article according to claim 1, furthermore comprising at least one monolayer of unidirectional fibers.

7. The article according to claim 1, further comprising at least two layers of unidirectional aligned polyethylene fibers.

8. The article according to claim 1, wherein the monolayer with unidirectional aligned fibers comprises a matrix material having a has a 100% modulus of at least 3 MPa.

9. The article according to claim 8, wherein the matrix material is a thermoplastic polyurethane.

10. The article according to claim 1, wherein the article has an areal weight between 75 and 1250 gram per square meter.

11. The article according to claim 1, wherein the article has an areal weight between 125 and 1000 gram per square meter.

12. The article of claim 1, wherein the article is a helmet.

13. The article of claim 1, wherein the article is radome.

14. Process for the manufacture of a three dimensional shaped article having an outer and inner surface, comprising the steps of (a) providing at least one fabric of polyethylene fibers having a tensile strength of at least 1.5 GPa, the fabric is impregnated with an acrylic based thermoplastic material, whereby at least one fabric forms the outer layer of the article, and (b) stacking the product from (a) with at least one optional monolayer of unidirectional aligned fibers (c) providing a mold for shaping the 3 dimensional article, (d) optionally coating the mold surface with a mold release agent (e) positioning the stack of step (b) in the mold, followed by (f) compressing the stack at a temperature between 90 and 135 C., at a pressure between 1 and 35 MPa, during a time of between 2 and 60 minutes, followed by (g) cooling to a temperature below 80 C., and releasing the so obtained article from the mold.

15. Use of a fabric with polyethylene fibers with an acrylic based thermoplastic material as outer surface for the manufacture of three dimensionally shaped articles including helmets.

Description

[0047] In the drawings, FIG. 1 represents a graphic of a cross section of a three dimensional shaped article having an outer (1) and inner (2) surface, the outer surface comprising at least one fabric (100) comprising polyethylene fibers.

TEST PROCEDURES

[0048] Polymer number average molecular weight of the acrylic thermoplastic material is determined by gel permeation chromatography according DIN 55672 at 40 C., with tetrahydrofuran as solvent, styrene/divinyl bezene as packing material and calibrated using Polystyrene Mp 160-10,000,000 (polymer standard service (PSS) DIN certified as standard.

[0049] The glass transition temperatures of the polymers in the examples use the values in C. determined experimentally using differential scanning calorimetry DSC (10 C/min), taking the peak of the derivative curve as Tg.

[0050] The modulus of the matrix material was determined according to ISO 527. The 100% modulus was determined on film strips with a length of 100 mm (free length between the clamps) and a width of 24 mm. The 100% modulus is the secant modulus measured between strains of 0% and 100%.

[0051] Tensile strength (or strength), are defined and determined on multifilament yarns as specified in ASTM D885M, measured at 25 C. using a nominal gauge length of the fiber of 500 mm, a crosshead speed of 50%/min. On the basis of the measured stress-strain curve the modulus is determined as the gradient between 0.3 and 1% strain. For calculation of the modulus and strength, the tensile forces measured are divided by the titre, as determined by weighing 10 meters of fiber; values in GPa are calculated assuming a density of polyethylene of 0.97 g/cm.sup.3.

[0052] Intrinsic Viscosity (IV) of polyethylene 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;

[0053] Adhesion testing was done with the Gitterschnitt test per ISO 2409 The test specimen is scratched in a regular pattern at a scratch distance of 1 mm whereby the scratches must be in the coating not in the substrate. A 3M adhesion tape (Scotch) is applied on the scratch pattern and pulled off subsequently. Good adhesion means that the coating will not come off. Poor adhesion will result in separation of the coating from the substrate.

[0054] The amount of separation is visually quantified.

[0055] The invention will now be further elucidated with the following comparative experiments and Example, without being limited hereto.

EXAMPLES

Materials:

[0056] Fabric: a plain woven fabric with Dyneema UHMWPE fibers with a strength of 3.5 GPa and with 30 wt. % of Neocryl (methylmethacrylate acrylic copolymer); total weight of the sheet was 245 g/m.sup.2
CF: a plain woven structure of polyethylene fibers in one direction and carbon fibers in the opposite direction, with 31 wt % of polyethylene; total weight of one sheet was 235 g/m.sup.2
UD: one sheet consisting of layers of cross plied monolayers with Dyneema UHMWPE fibers with a strength of 3.5 GPa and 18 wt % of polyurethane resin based on a polyetherdiol and an aliphatic diisocyanate; total weight of the sheet was 145 g/m.sup.2

Comparative Experiment A

[0057] A helmet was produced by stacking 43 sheets UD and pressing these in a mold at a pressure of 175 bar and a temperature of 130 C. during 25 minutes, followed by cooling under pressure to at least 80 C. before releasing from the mold. The mold was sprayed with a mold release agent, before placing the stack in the mold.

[0058] After cutting debris from the rim of the helmet, some surface defects occurred due to delamination of some filaments at the outer surface, from the cut rim. Furthermore some folds occurred in the outer surface layer due to the molding process. The helmet was coated with a standard green paint and the surface appearance was visually checked. It was seen that the surface defects could not be mitigated by the paint, the defects still were visible at the outer surface.

[0059] Paint adhesion was tested via Gitterschnitt, and proved poor due to chipping-off of paint.

Comparative Experiment B

[0060] A helmet was produced in the same was as Comparative experiment A, with 42 sheets UD and one outer layer of CF were pressed.

[0061] No folds occurred in the outer surface layer of the helmet; however fiber breakage of the carbon fibers in the CF occurred due to high shear forces in the more vertical part of the helmet. After painting the surface appearance of the helmet was flawed due to the still visible broken fibers in the CF outer layer.

[0062] Paint adhesion via Gitterschnitt proved good, no chipping-off of paint occurred.

Example 1

[0063] A helmet was produced in the same was as Comparative experiment A, 42 sheets UD and one outer layer of fabric were pressed.

[0064] No folds occurred in the outer surface layer of the helmet after pressing; no fiber breakage occurred and the surface was smooth. After painting the surface appearance smooth without defects.

[0065] Paint adhesion via Gitterschnitt proved good, no chipping-off of paint.

[0066] Only the article according to the invention, as exemplified in Example 1, showed a good surface appearance as can be judged by the smooth surface substantially without wrinkles, before and after painting, as well as good paint adhesion.