Puncture resistant material

Abstract

A puncture resistant material is made from high modulus continuous filament polypropylene yarns which are twisted and woven into a tight weave. Batting materials are placed adjacent the woven layer (which may comprise one or more individual woven layers) to form a stack and the stack is needlepunched to form a consolidated material. The material is heat treated and calendared and the finished product may be used in applications where puncture resistance is required, such as in a shoe insole material.

Claims

1. A flexible puncture-resistant material, comprising: a woven layer comprised of tightly woven yarns made from twisted high modulus continuous filament polypropylene; said woven layer having 1-10 oz/yd.sup.2 thermoplastic batting fibers needlepunched in a perpendicular direction into the woven layer forming a consolidated material; said consolidated material being (i) condensed by heat treatment at a temperature above a glass transition temperature of the thermoplastic batting fibers to partially melt but not completely melt the thermoplastic batting fibers, thereby thermally shrinking the thermoplastic fibers, and (ii) reduced in thickness by calendaring.

2. The material according to claim 1, wherein the thermoplastic yarns are twisted at 1-5 turns per inch prior to weaving.

3. The material according to claim 1, wherein the woven layer is a 2020 to 3030 plain weave.

4. The material according to claim 1, wherein the woven layer is a plain weave with greater than about 90% cover factor.

5. The material according to claim 1, wherein the woven layer comprises 1 to 20 individual woven layers.

6. The material according to claim 1, wherein the batting fibers consist of about 1 to about 10 oz/yd.sup.2 of a blend of thermoplastic fibers.

7. The material according to claim 1, wherein the consolidated material has a density increased by about 5% to about 20% compared to the consolidated material prior to heat treatment.

8. The material according to claim 1, wherein the consolidated material has a density increased by about 10 to 12% compared to the consolidated material prior to heat treatment.

9. The material according to claim 1, having a thickness of about 0.10 to about 0.20 inches.

10. The material according to claim 1, having a thickness of about 0.05 to about 0.50 inches.

11. The material according to claim 1, meeting ASTM 2412/2413 for puncture resistance.

12. A shoe sole comprising an insole made of the puncture resistant material according to claim 1.

13. The material according to claim 1, wherein said thermoplastic batting material fibers are made of the same high modulus continuous filament polypropylene as the woven layer and the fibers of the woven layer are partially but not completely melted.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The FIGURE is a graphic depicting the performance of a shoe sole material in a puncture resistance test according to the present invention, compared to a steel plate.

DETAILED DESCRIPTION OF THE INVENTION

(2) Standards for protective footwear are described in ASTM F2413, entitled Standard Specification for Performance Requirements for Protective (Safety) Toe Cap Footwear and ASTM F2412, entitled Standard Test Methods for Foot Protection. Among other criteria, these standards set forth puncture resistance and flexibility requirements for a shoe sole material. Reference to a particular ASTM standard herein means the standard in effect on the effective filing date of this application. These standards set forth testing protocols for shoe sole materials generally recognized by those of ordinary skill in the art and are incorporated by reference.

(3) The puncture resistant shoe sole material of the present invention is preferably used inside of an outermost layer of a shoe sole, i.e., as an insole material and is preferably in an intermediate layer, between an outer layer and an inner layer adjacent the wearer's foot.

(4) The woven component of the material is formed from high strength polypropylene fibers.

(5) To form the woven layer, the yarns are first twisted from 1 to 5 turns per inch to allow the material to be woven fairly tightly and act more like a monofilament. As an example, twisting may increase the denier of a polypropylene fiber yarn from 940d to 1150d. However, these results depend greatly on the denier of the starting material.

(6) The yarns are woven to create a woven layer having a fairly tight weave, generally having greater than about 80% cover factor, and preferably greater than about 90% cover factor. A 2525 plain weave has been found suitable, but is not critical. A 2020 weave to about 3030 weave is contemplated. In the embodiment of Example 1, a 2424 weave is used.

(7) From 1 to 20 woven layers, and preferably 5 to 9 woven layers are positioned with fiber batting material on one or both sides to form a stack. The stack is consolidated by needlepunching so that the batting material is forced into the interstices of the woven layer and forms a generally monolithic material.

(8) The fiber batting material consolidated with the woven layers may be a batt of the same fibers used in the woven layers, or may be different. It is preferred to use high strength thermoplastic fibers, such as, without limitation, certain thermoplastic polyester fibers, polyamide fibers, poly(arylene sulfide) fibers; and high strength polypropylene and polyethylene fibers, all of which are available commercially and well characterized as to strength. Preferably 1-10 oz/yd.sup.2 of batting fibers are used. In some instances, 5-8 oz/yd.sup.2 has been found to be appropriate.

(9) Heat treatment is conducted to increase the density of the consolidated material. Preferably, heat treatment is conducted to a temperature slightly above the glass transition temperature of the thermoplastic fibers to partially melt the thermoplastic fibers. The thermoplastic fibers are not substantially completely melted to form a matrix, as might be expected forming a fiber composite, but the thermal shrinkage tightens the weave even more and partially melts the through thickness fibers to bond the system together. The density of the material is preferably increased in a range of about 5% to about 20%. In some cases, it is desired to increase the density in a range of about 8 to about 15%. In presently preferred, but non-limiting embodiments, the density is increased about 10 to 12%. In the case of certain polypropylene fibers, the heating step is preferably conducted in a dryer at a temperature in the neighborhood of 320 F.

(10) Calendaring further increases the density of the material and reduces the thickness of the material to about 0.05 to about 0.35 inches, preferably about 0.10 to about 0.20 inches, which is considered suitable for most shoe insole applications. The weight of the insole material is preferably 50-120 oz/yd.sup.2, more preferably 65-75 oz/yd.sup.2.

(11) The material is incorporated into a protective shoe by cutting the puncture resistant material to form an insole profile covering substantially the inside bottom surface of a shoe to form a puncture-resistant barrier. Preferably, the material is an insole material, intermediate an inner layer and an outer sole layer.

Example 1

(12) Highly drawn and high strength INNEGRA brand polypropylene continuous filament yarns having a denier of about 940d were twisted fairly aggressivelyabout 2.5 turns per inch (TPI)and woven into an approximately 2424 plain weave. Six of the woven layers were arranged in a stack, and a batt of similar polypropylene fibers was arranged on the top and bottom of the stack and consolidated with the woven layer by needlepunching. The consolidated material was heat set in an oven at 320 F. to shrink the materials significantly. The resulting material was calendared under heat and pressure low enough not to significantly impact the physical properties of the fibers, but sufficient to reduce the thickness and smooth the surface. Before arranging in a shoe insole, the resulting material has a thickness of about 0.170 inches, a weight of about 67.5 oz/yd.sup.2 and passes ASTM 2412/13 for puncture resistance. Arranged as an intermediate layer in a shoe sole, the resulting product passes ASTM 2412/13 for flexibility.

(13) FIG. 2 shows the penetration resistance of a material according to Example 1 compared against a standard steel shoe sole material having a thickness selected to pass ASTM 2412/13.

(14) The above description of the preferred embodiments is not to be deemed limiting of the invention, which is defined by the following claims. The foregoing description should provide the artisan of ordinary skill with sufficient information to practice variants of the embodiments described. Features and improvements described in connection with one embodiment or one claim may be combined with other embodiments or other claims without departing from the scope of the invention.