Inextensible Filamentary Structure And Fabrics Woven Therefrom

Abstract

Strands of brittle, highly inextensible filamentary material are collimated into a bundle and spot bonded at regular intervals with an epoxy resin or the like, to form a composite yarn structure having sufficient flexibility and mechanical stability to permit the yarn to be woven as the warp of a fabric, the fill yarn being any conventional fibrous material. The inextensible filamentary materials include, among others, boron, boron carbide, silicon, silicon carbide, carbon, quartz, and similar inorganic refractory fibers which are characterized by high strength and modulus, brittleness, and inextensibility.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of weaving fabrics from brittle and highly inextensible filamentary materials. More particularly, this invention relates to a method for forming a composite yarn structure from inorganic refractory fibers and utilizing this composite as the warp yarn in weaving a fabric.

2. Description of Prior Art

Recent developments in the field of high strength, temperature resistant materials have included the use of inorganic refractory fibers such as boron, boron nitride, boron carbide, silicon, silicon carbide, alumina, alumina-silica, carbon, glass, and quartz to fabricate reinforced composite structures having outstanding stiffness and strength-to-weight ratios. These inorganic refractory fibers are characterized by high strength modulus, brittleness, limited flexibility, and by essentially zero elongation as defined by the term "inextensible."

Although many of these refractory fibers are presently available only in short lengths or as whiskers, efforts are being made to produce the materials in continuous filament form. Continuous filament boron formed by the vapor deposition of boron on a fine wire tungsten substrate is now commercially available.

High-strength reinforced structures are generally fabricated either by filament winding or by sheet layup. In filament winding, strands of reinforcement are oriented directly onto surfaces which control their form. In sheet layup, the strands are first formed into sheets, and the sheets are laid up and laminated to provide the desired form and orientation. While these methods are useful, composites made by these processes do not possess the cross-reinforcement provided by a woven structure. For this reason it is desirable to prepare woven fabrics of the inextensible materials as reinforcement for composite structures.

Although the art of weaving is ancient, the inextensible and brittle nature of the inorganic refractory fibers makes it extremely difficult to weave these materials in a conventional manner. Conventional textile yarns have a reasonable amount of elongation due to the intrinsic properties of the fibers used and due to the generally twisted yarn structure. Consequently, such yarns are capable of absorbing energy to fairly high strain levels, without breaking. They may also be bent to very short radii of curvature without damage.

Filaments of the inorganic refractory fibers presently available conform to none of these conditions. The high filament modulus, in combination with a relatively large filament diameter, results in a gross stiffness and consequent fabrication difficulties. Since the bending moment of a circular rod is proportional to the fourth power of the radius of the rod, it is apparent that the intrinsic stiffness of the material is augmented by its geometry. Attempts at weaving individual monofilaments into the warp of a fabric have resulted in frequent breakage of the brittle filaments. A method for handling and weaving these materials was clearly needed if the advantages of the woven fabric in reinforced composited structures were to be obtained.

SUMMARY OF THE INVENTION

It has been discovered that a multiplicity of high modulus, high strength, brittle, and highly inextensible filaments may be collimated into a bundle and spot bonded at regularly spaced intervals to hold the bundle as a unit for further processing. The preferred bonding agent is an epoxy adhesive or cement, although any of several suitable resins may be employed.

The inextensible filaments which may be used in the practice of this invention are the inorganic refractory fibers which have a tensile strength of at least 50,000 p.s.i. (pounds per square inch) and an elastic modulus of at least 4 million p.s.i. Included among others are boron, boron nitride, boron carbide, silicon, silicon carbide, alumina, alumina-silica, carbon, and quartz. The filament may also be a composite structure consisting for example of a tungsten wire core with a sheath of boron, boron carbide, or titanium diboride.

The bonded bundles can be handled with relative ease and can be woven into tapes, ribbons, or fabrics using the bundles as warp ends. The advantage of the bundle structure arises from the freedom of the component filaments to move with respect to the others, particularly in bending, and to assume a position of minimum strain. Furthermore, should one or more individual filaments be broken, the unity of the bundle is maintained by the points of bonding.

The frequency of the spot bonding intervals can be varied to suit filament length. Thus, for bonding long continuous strands of filamentary material, the spot may be spaced several inches apart, while for bonding shorter lengths, the spots may be only one inch or less apart.

As stated above, epoxy resins are preferred as the bonding agent because of their high strength and ability to dry and cure into a hard, smooth, nontacky bond. Other bonding agents in the form of cements, adhesives, etc. which can be used with good results include, for example, acetate, polyester, polyamide, polyimide, phenolic alkyd, and acrylic resins. Generally, it is preferred to select the bonding agent to be compatible with the intended end use of the final inextensible fabric. In other words, if the final fabric is to be used to form an epoxy bonded laminate structure, where several layers of the fabric are impregnated with an epoxy resin and cured while held in a desired structural configuration, then it would be logical and desirable to use the same or similar epoxy resin as the spot bonding agent in forming the original composite yarn structure.

In some instances, where the presence of a foreign substance may be undesirable in the final fabric, the bonded yarn may be woven into the fabric and the bonding material subsequently removed by washing with a solvent.

Collimated bundle fabrics made from high modulus, high strength, brittle, and highly inextensible filaments may be incorporated in resin matrices to produce high performance composite. The reinforcing filaments are present in high density, and their intrinsic filament properties are unimpaired by the geometry of the fabric. Therefore, the composite has a high strength to weight ratio, high modulus, and high stiffness.

Filling yarns for weaving the collimated bundle warp fabric may be selected from a wide variety of available materials including glass, cotton, wool, and organic manmade fibers. If the transverse properties of the fabric reinforcement in the ultimate composite are not critical, it is convenient to use synthetic organic fiber yarns such as nylon, polyester, the acrylics, or polyolefin. If, however, the transverse properties are of consequence, high performance synthetic organic fibers, glass, quartz and other available materials may be used.

DESCRIPTION OF THE DRAWING

FIGS. 1, 2 and 3 show three typical cross-sectional views of the spot bonded composite yarn structure. The inextensible filaments are designated by the numeral 10, while the bonding material is designated by numeral 12.

FIG. 4 shows a representative side view of a spot bonded composite yarn structure produced in accordance with the instant invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the practice of the present invention, a plurality of the inextensible inorganic refractory fibers are collimated into a bundle and bonded together in a parallel and substantially untwisted relationship by spot bonding the bundle with an adhesive material at regular intervals to maintain its integrity. Generally, the bundle will be comprised of from three to seven essentially continuous inextensible monofilaments of 100 yards or greater length. The bundle may also be comprised of an arrangement of an inextensible filaments having shorter lengths of from about 2 to about 8 inches or longer. Generally, 2 inches is the practical minimum length which can be used in accordance with this invention.

For ease of handling, and for superior product characteristics, the continuous monofilaments are preferred. Excellent results are obtained using the commercially available filaments produced by vapor deposition of boron on the very fine tungsten wire substrate. These filaments have a tensile strength of about 400,000 p.s.i., an elastic modulus of about 60 .times. 10.sup.6 p.s.i., and an upper temperature limit of about 2000.degree. C. in an inert atmosphere. Three to seven of these composite monofilaments bonded at regular intervals of about 6 to 12 inches or so are easily handled and can be used as warp ends to weave a fabric having in the warp direction the same high strength and modulus as the boron-tungsten filaments.

The bonding material may be applied to the filaments by hand or automatically by machine. It is only required that provision be made to first of all collimate the filaments into a compact bundle, and second, to hold the bundle in a fixed relationship while the adhesive is applied and cured.

The bonded composite yarns can be handled with relative ease and with little danger of breaking any of the inextensible filaments if reasonable precautions are taken to avoid folding or bending the yarn about a very small radius. Although the composite yarns are generally used as warp ends in constructing the fabric, they may also be used as fill if it is preferred. In this case, the inextensible filaments are most conveniently cut to a length corresponding to the width of the fabric, and individually placed through the warp shed during the weaving process. When using the inextensible filaments as fill, it is generally advisable to use a flexible textile yarn as the warp.

The following examples will serve to further illustrate the invention.

EXAMPLE 1

Three ends of continuous boron-tungsten sheath-core composite monofilament 8 mils in diameter were collimated into a bundle and spot bonded with drops of epoxy cement applied at 12 inch intervals. The epoxy bonds were dried and cured by heating with a stream of hot air. A woven fabric was prepared by using 80 bundles of the bonded composite yarn per inch of width as warp ends and a single 189 denier fiberglass yarn as the fill. The resulting fabric was completely flexible in the fill direction and sufficiently flexible in the warp direction to bend around a 2-inch radius material with no fracture of boron filaments.

EXAMPLE 11

Seven boron-tungsten monofilaments were spot bonded at 6 inch intervals with an epoxy resin according to the procedure of Example I. The composite was very flexible and easily woven into a fabric having 40 braid covered bundles per inch as warp ends with a single 189 denier fiberglass yarn as fill. The lightweight fabric had excellent flexibility.

It will be apparent from the foregoing description and examples that this invention provides a novel method for preparing yarns and fabrics from highly inextensible filamentary materials. As many widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not to be limited to the specific embodiments thereof except as defined in the appended claims.

Claims

I claim:

1. A method of preparing a composite multifilament structure of inorganic inextensible fibers characterized by high strength modulus, brittleness, limited flexibility and essentially zero elongation suitable for use as the warp yarn in weaving inextensible fabrics which comprises:

a. collimating a plurality of inextensible filaments into a compact bundle in a parallel and substantially untwisted relationship to form a continuous filamentary structure, said filaments having a tensile strength of at least 50 thousand p.s.i. and an elastic modulus of at least 4 million p.s.i., said filaments being a composite filament comprised of tungsten wire core and a sheath selected from the group consisting of boron and boron carbide,

b. applying a bonding agent to the surface of the filaments in spots at regular intervals along the longitudinal axis of the continuous filamentary structure, said bonding agent being a member selected from the group consisting of acetate, polyester, polyamide, polyimide, phenolic alkyd acrylic and epoxy resins, and

c. maintaining the compact bundle of inextensible filaments in a fixed position until the bonding agent is hardened and the integrity of the filament bundle is assured.

2. The method of claim 1 wherein the bonding agent is applied at intervals of from about 0.5 to about 12 inches along the longitudinal axis of the continuous filamentary structure.

3. The method of claim 1 wherein the number of inextensible filaments comprising the bundle is from three to about seven.

4. A composite filamentary structure comprised of a plurality of substantially parallel, substantially untwisted and closely compacted highly inextensible monofilaments bonded in spots at regular intervals along the length thereof with an adhesive material, said filaments having a tensile strength of at least 50 thousand p.s.i. and an elastic modulus of at least 4 million p.s.i., said filaments being a composite filament comprised of tungsten wire core and a sheath selected from the group consisting of boron and boron carbide; said adhesive material being a member selected from the group consisting of acetate, polyester, polyamide, polyimide, phenolic alkyd and epoxy resins.

5. A woven fabric having fill yarns and warp yarns, said fill yarns being a flexible textile material, said fill yarns being selected from the group consisting of glass, cotton, wool and organic manmade fibers, and each of said warp yarns being a composite filamentary structure comprised of a plurality of substantially parallel, substantially untwisted and closely compacted highly inextensible monofilaments bonded in spots at regular intervals along the length thereof with an adhesive material, said filaments having a tensile strength of at least 50 thousand p.s.i. and an elastic modulus of at least 4 million p.s.i., said filaments being a composite filament comprised of tungsten wire core and a sheath selected from the group consisting of boron and boron carbide, said adhesive material being a member selected from the group consisting of acetate, polyester, polyamide, polyimide, phenolic alkyd, acrylic and epoxy resins.