JP3125222B2 - Fibrous product and method for producing the same - Google Patents

Description

The present invention relates to a fibrous product and a method for producing the fibrous product.

The composite includes a matrix that includes reinforcing fibers and is supported by the reinforcing fibers. The reinforcing fibers usually have a high tensile strength and / or tensile modulus. Examples of reinforcing fibers include carbon fiber and aramid fiber (EIDuPont d
e Nemours & Co., commercially available under the trademark Kevlar), highly drawn polyethylene fibers (Allied-Si
gnal Corp. under the trademark Spectra), and polybenzoazole fibers.

Fibers with high tensile strength typically have relatively low compressive strength. The compressive strength of fibers with high tensile strength rarely exceeds 30 percent of the tensile strength, and is often much lower. It also has a very low compression set-failure ratio, requiring little effort to cause compression failure in the fiber. The poor compression properties of reinforcing fibers significantly reduce their effectiveness in matrix composites for many structural applications.

Many attempts have been made to improve the compressive strength of drawn polymer fibers. For example, Arnold, “Structural M
odifications of Rigid-Rod Polymers, "The Materials
Science and Engineering of Rigid Rod Polymers 11
7,121-122 (1989), the polymer in the fiber was crosslinked. However, this polymer-based method did not yield fibers having a compressive strength at least 100 percent higher than the compressive strength of the uncrosslinked fibers.

Antal et al., Reinforcement Structure, U.S. Pat.
No. 9,716 (February 29, 1985) discloses a thick core of high tenacity fiber, which solidifies into bars by impregnating and curing with an epoxy resin prior to packaging, where the core has a radius of at least 0.1 percent. It teaches that compressive strength is improved by wrapping in a spiral winding of a high tenacity yarn under very high tension such that it is compressed in the direction. Flexibility is desired so that the fibers are formed into a desired shape before being wrapped on a spool and cured into a composite.

Enhancing the compressive strength of fibers or matrix composites containing fibers and / or matrix composites containing fibers or fibers, while leaving the fibers sufficiently drapeable and handleable before curing into the composite It is necessary to increase the amount of effort required to cause compression failure.

A first aspect of the present invention provides a method comprising: (a) a core comprising one or more essentially parallel core fibers; (b) one or more packaging fibers surrounding the core and covering at least 50 percent of the outer surface of the core. And (c) a curable resin which is flowable before curing and can be cured to a cured resin having a compression modulus of at least 50,000 psi; (1) the core has an average diameter of 0.8 mm or less; And the compressive strength of the core fibers contained in the core is no more than 30 percent of its tensile strength; (2) the wrapping fibers surround the core under radial compression of less than 0.1 percent; and (3) the curing A fibrous product characterized in that a conductive resin impregnates both the core fiber and the packaging fiber.

The second embodiment of the present invention comprises the following steps: (1) 21 g to 1000 g around a core having an average diameter of 0.8 mm or less, including a large number of core fibers whose compressive strength is 30% or less of its tensile strength. Wrapping the packaging fibers with a tension of to form a fibrous product; and (2) flowable and at least 50,000 psi before curing.
Impregnating the fibrous product with a curable resin that can be cured into a cured resin having a compression modulus of, and impregnating both the core fiber and the wrapping fiber with the curable resin. is there.

The method of the present invention can be used for producing the fibrous products and prepregs of the present invention. The fibrous product and prepreg are sufficiently flexible to be drapeable. This prepreg is useful in the manufacture of composites that are molded before curing to form an effective structural material. The composite preferably has a compressive strength that is at least 10 percent higher than the compressive strength of a similar composite made using only the core fibers. This preferably requires substantially more effort to cause compression failure than composites containing only unpackaged core.

The present invention uses a core containing reinforcing fibers. The reinforcing fibers preferably have a tensile strength of at least 2 GPa, more preferably at least 3 GPa, most preferably at least 4 GPa. It preferably has a tensile modulus of at least 100 GPa, more preferably at least 200 GPa.

The compressive strength of a reinforcing fiber is less than 30 percent of its tensile strength. It is usually less than 1 GPa, less than 0.5 GPa or
It may be less than 0.30 GPa. The polymer is preferably aramid, highly oriented polyethylene or polybenzoazole. It is more preferably aramid or polybenzoazole, most preferably polybenzoazole. Suitable fibers are described in detail below.

Aramid fibers are known and are commercially available. Suitable fibers are commercially available under the trademarks Kevlar, Twaron and Technora. The polymer in the fiber preferably contains predominantly p-phenylene moieties linked by amide units. Preferred polymers are m- and p-
Although containing a mixture of phenylene moieties, most preferred polymers are essentially free of m-phenylene moieties. Aramid fiber is available from 3Kirk-Othmer Ency. Chem.Tech. (3rd edition), Ara
mid Fibers, 213 (J. Wiley & Sons 1978).

Stretched polyethylene fibers are also known and commercially available. Stretched polyethylene fibers are typically
Gel spun ultra high molecular weight polyethylene. Suitable fibers are commercially available from Allied Signal Co. under the trademark Spectra.

Polybenzoazole polymers and methods for producing fibers therefrom are also known. The polybenzoazole polymer contains (1) an aromatic group (Ar), and (2) a first azole ring condensed with an aromatic group, and preferably further (3) a first azole ring condensed with the first aromatic group. A second azole ring, and (4) a number of monomer units including a divalent organic moiety (DM) that does not interfere with the use or synthetic processing of the fiber attached to the 2-carbon of the second azole ring.

The polybenzoazole monomer unit preferably has the following formula 1
It is represented by one of (a) or (b), and more preferably by Formula 1 (b).

(In the above formula, each Ar represents an aromatic group, and each Z is -O-, -S
-Or -NR-, where each R represents a hydrogen atom, a lower alkyl group or a phenylene moiety, and each DM represents a bond or a divalent organic moiety as described above.) Each aromatic group (Ar) is It is preferably a carbocyclic group containing not more than 12 carbon atoms, and more preferably, in the case of AB-polybenzoazole (AB-PBZ: formula 1 (a)), a 1,3,4-phenylene moiety or AA / BB- In the case of polybenzoazole (AA / BB-PBZ: formula 1 (b)), it is a 1,2,4,5-phenylene moiety. Each azole ring is preferably an oxazole or thiazole ring (-Z-=-O- or -S-), and more preferably an oxazole ring (-Z-=-O-). Each DM is preferably an aromatic group, more preferably a 1,4-phenylene moiety. Said moieties are preferably chosen such that the resulting polymer is a rigid rod polymer or semi-rigid polymer, more preferably the resulting polymer is a rigid rod polymer. Highly preferred monomer units are represented by formulas 2 (a)-(e).

Polybenzazole polymers are disclosed in U.S. Pat.
No. 3, essentially 1 as described in columns 9-45
A polybenzoazole "homopolymer" consisting of three repeating monomer units, or U.S. Pat.
No. 693, columns 45-81 and Harris et al., Polymer Contain.
ing Polybenzoxazole, Polybenzothiazole and Polybenz
imidazole Moieties, International application PCT / US89 / 04464 (1989 January 1)
(Published on June 6, 1990) and WO 90/03995 (published on April 19, 1990). This polymer is preferably a "homopolymer". This is more preferably
Forming a liquid crystal solution when dissolved at an appropriate concentration in a solvent acid such as polyphosphoric acid and / or methanesulfonic acid; and / or
Alternatively, it coagulates from a solvent acid to form crystalline or semi-crystalline coagulated fibers.

The polybenzoazole polymer should preferably have a sufficient molecular weight to form a spinnable dope solution. Its molecular weight is preferably at least 5000, more preferably at least 10,000, most preferably at least 2
5,000. For poly (paraphenylene-cis-benzobis-oxazole) (cis-PBO), the intrinsic viscosity of this polymer in methanesulfonic acid at 25 ° C. and 0.05 g / dL concentration is preferably at least 10 dL / g, more preferably at least 10 dL / g. 20 dL / g, most preferably at least 30 dL / g
It is. This is preferably less than 50 dL / g.

Polybenzazole polymers, dehydrated acid solution such as polyphosphoric acid and / or mixtures of methanesulfonic acid and P ₂ O _5, is synthesized by the reaction of suitable monomers with vigorous stirring under nitrogen atmosphere. The reaction temperature is typically between 75 and 220 ° C. and usually increases in two stages. The resulting dope is then spun and drawn into a suitable coagulation bath by dry jet wet spinning to form fibers. The synthesis and fiber spinning of suitable polybenzoazole polymers is described in many references, for example, U.S. Patent Nos. 4,263,245, 4,533,693 and 4,776,678;
CT International Publication WO 90/03995 (published April 19, 1990) and L
edbetter et al., “An Integrated Laboratory Process for
Preparing Rigid Fibers from the Monomers, "The Mat
erials Science & Engineering of Rigid Rod Polymer
s 253 (Materials Research Society 1989).

The spun polybenzazole fibers are exposed to somewhat elevated temperatures under tension ("heat treatment" or "heat conditioning") to improve the tensile strength and / or modulus as described in U.S. Patent No. 4,544,119. )). The heat treatment may be at a temperature of 300-700 ° C. for a period of a few seconds to 30 minutes.
Of course, a long time is desirable at low temperatures, and a short time is desirable at high temperatures.

The reinforcing fiber in the present invention is constituted by a core containing at least one type of reinforcing fiber. The core may include one fiber, but preferably includes multiple fibers. The core fibers are preferably parallel to each other, more preferably at least some of the fibers in the core are substantially untwisted but extend parallel to the longitudinal axis of the fibrous product. The core may include two or more fibers, such as a mixture of aramid fibers and polybenzoazole fibers.

The maximum and minimum size of the core is determined mainly from a practical standpoint. Cores with too small an average diameter are undesirable for at least two reasons. First, cores containing only one fiber are too thin and difficult to package by wrapping with packaging fiber unless the packaging fiber is very flexible.
Second, very thin cores have a high sheath to core fiber ratio. To obtain the best composite properties, it is desirable to minimize the ratio of sheath to core fiber. On the other hand, cores with too large an average diameter are also undesirable. This is because thick cores are usually substantially less flexible than thin cores.

The core has an average diameter of 0.8 mm or less. The average diameter is preferably at most 0.6 mm, more preferably at most 0.5 mm, most preferably at most 0.4 mm. The average diameter of the core is preferably at least 0.005 mm, more preferably at least
0.1 mm. When the fiber is an aramid fiber such as Kevlar fiber, the core preferably has a denier of 3000 or less, more preferably 2500 or less, most preferably 1500 or less, preferably at least 200, more preferably at least 500, most preferably Has a denier of at least 1000.

The core is covered with an exterior containing packaging fibers surrounding the core. The packaging fiber should be flexible enough to wrap tightly around the core without substantial damage. The packaging fiber preferably has a high glass transition temperature and sufficient thermal stability that allows its use in most temperature ranges where the core fiber is effective. Suitable packaging fibers include, for example, polybenzoazole, aramid,
Including nylon, polyester, polypropylene, or polyethylene. Preferred packaging materials are polybenzoazole fibers and aramid fibers. The polybenzoazole is preferably not a rigid rod polybenzoazole, but is preferably AB-polybenzoazole or a soft coil AA / BB
-A polybenzoazole polymer.

The wrapping fiber is wound around the core using a number of known devices. Examples of methods of packaging fibers around fibers, and the products of this method, are described in many references, such as U.S. Pat.
495,646, 3,556,922, 3,644,866, 4,269,024,
4,272,950, 4,299,884, 4,384,449, 4,499,716
No. 4,861,575.

If the core is very thin, such as a single fiber, it is difficult to wrap the core with solidified fibers without damaging the core. Instead, the core is wrapped with fibers of a flowable and handleable material, such as wrapping a polybenzoazole-containing or aramid-containing acid-doped strand or molten nylon around the core. The flowable package is then solidified by coagulation in the case of a dope or by cooling in the case of a molten polymer. The flowable package should be sufficiently viscous to retain its shape until solidified. The flowable packaging is preferably a polybenzoazole dope.

The armor should be thick enough to provide radial confining pressure without breaking. However, the exterior is preferably as thin as possible for two reasons. First, a thicker sheath increases the overall thickness of the fibrous product. Thick fibrous products have low flexibility, low drapability and poor handling. Secondly,
When the fibrous product contains a high ratio of core fibers to the sheath, high composite compression and tensile strength is achieved. This is theorized that the axial strength of the fibrous product in tension and compression is obtained from the core rather than from the armor. If the cladding occupies a large portion of the volume allowed for the fibrous product in the composite, a small amount of this volume core fiber must be included. By manufacturing the sheath as thin as possible, the amount of core fibers and the strength of the resulting composite are maximized.

The actual thickness is mainly governed by the packaging fiber strength and flexibility. The armor preferably has a thickness of 0.2 mm or less, more preferably 0.15 mm or less, and most preferably 0.1 mm or less.
An armor containing packaging fibers has one, two or more packaging layers, but preferably comprises two or more layers, more preferably one or more layers.

The packaging fibers need not cover 100% of the outer surface of the core. The packaging fiber is preferably at least 70% of the core surface
Percent, more preferably at least 90 percent,
Most preferably, it covers 100 percent.

The wrapping fibers are preferably wound around the core by tension. Preferably, the packaging mechanism used to package the core has a tension generating means such as a brake or clutch for the packaging fibers. Otherwise, Leesona Co
Tension is generated by packaging at high speed in a hollow core spindle device such as a verspun. If the packaging device does not include a tension generator, the speed of packaging is preferably at least 15,000 laps / min, more preferably at least 3 laps / min.
It is 0,000 lap / min.

The tension on the packaging fibers is preferably at least 20 g, more preferably at least 50 g, most preferably at least 75 g. Very high tensions are neither necessary nor desirable. Wrapping the core under high tension will cause the core to twist or deform unless it is under high tension, and under high tension the core must be undesirably thick to prevent breakage. The tension of the packaging is preferably
It is 500 g or less, more preferably 260 g or less. The tension is suitable for packaging fibers having a diameter approximately comparable to 200 denier Kevlae aramid fibers. One skilled in the art would be able to adjust this tension relative to the other fibers to obtain substantially equal radial constraint pressure. Tension sufficient to compress the radial diameter of the core by 0.1 percent is undesirable and should be avoided.

The core is impregnated with a flowable, curable resin before packaging, and then the matrix resin is cured. The resin is preferably impregnated into the core after packaging the core. The flowable, curable resin may be a molten thermoplastic polymer such as poly (aromatic ether ketone), poly (aromatic ether sulfone) and poly (ether imide).
The fluid, curable resin is preferably an epoxy resin,
Thermosetting resins such as polycyanate resin, phenol resin, butadiene resin, vinyl ester resin and polyimide. The thermosetting resin is preferably an epoxy resin or a polycyanate resin. The flowable, curable resin preferably has a compression modulus after curing of at least 50,000 psi, more preferably at least 100,000 psi, and most preferably at least 250,000 psi.

The flowable and curable resin should not be completely cured before packaging, but may be partially cured as long as the fibrous product is flexible. The packaged fibrous product is harder and less manageable after the resin has cured. Thus, the resin should not be fully cured until the armor has been applied to the core and, preferably, the matrix composite containing the fibrous product has cured.

Preferably, the diameter of the fibrous product, including the armor and the core, is small enough that the fibrous product is flexible, drapable and handleable. This diameter is preferably 1.5mm
Or less, more preferably 0.8 mm or less, most preferably 0.6 mm
It is as follows. The minimum diameter of the fibrous product is governed by the size of the core fiber and the flexibility of the packaging fiber. The fibrous product preferably has a diameter of at least 0.1 mm, more preferably at least 0.3 mm.

The fibrous product is impregnated by a known method by impregnating the exterior and the core with a fluid, curable resin. This fluidity and curable resin has the same meaning and preferable characteristics as the core impregnating resin, and is preferably the same as the resin impregnating the core. The core is preferably impregnated before being sheathed, and the sheath of the fibrous product is preferably impregnated with the resin while adding the sheath to the core or in a separate step thereafter. If the resin is not added to the core before it is packaged, subsequent impregnation will not completely impregnate the core and the resulting composite may have poor compression properties.

The resulting prepreg should contain sufficient fluidity and curability to bend the fibers and allow the prepreg to cure with a number of other prepregs to form a matrix composite. The prepreg preferably contains sufficient matrix resin to minimize porosity in the fibrous product. In the present invention, it is desirable to maximize the volume percent of prepreg and composite occupied by the core fiber while maintaining radial pressure on the core fiber to fill the pores and maximize compression properties. The prepreg and the resulting matrix composite typically comprise a resin matrix.
25-60 volume percent and 40 fibers or fibrous products
Contains ~ 75 volume percent. The prepreg or composite more preferably comprises at least 60 volume percent fibrous product. It preferably contains no more than 20 volume percent, more preferably no more than 10 volume percent, even more preferably no more than 5 volume percent, and most preferably no more than 2 volume percent.

The impregnation and formation of matrix complexes has been described in many references, for example Kirk-Othmer Ency. Chem. Tech. Supplement, Composi.
tes, High Performance, 260-80 (J. Wiley & Sons 198
It is described in 4). The uncured prepreg is then laminated, draped and molded on a mold. The molded prepreg is cured by curing the thermosetting resin or by cooling the thermoplastic resin to form a molded product. The molded product is further processed to become a structural or electronic material.

Improvements in the compressive strength of fibrous products do not necessarily translate directly into improvements in composite properties. Core fibers do not make up 100 percent of the composite, even with unsheathed fibers, and sheathing reduces the volume of core fibers in the matrix. However, matrix composites made with fibrous products preferably have at least 10 percent higher compressive strength than those with unsheathed fibers. The improvement in compressive strength is more preferably at least 20 percent,
Even more preferably at least 50 percent, and most preferably at least 90 percent.

When the core fiber is a polybenzoazole polymer, the compressive strength of the composite containing the fibrous product is preferably at least 22 kpsi (151 MPa), more preferably at least
30kpsi (200MPa), most preferably at least 35kpsi
(240MPa). When the core fiber is aramid,
The compressive strength of the composite containing the fibrous product is preferably at least 30 kpsi (207 MPa), more preferably at least 35 kp
si (240MPa), most preferably at least 40kpsi (275M
Pa). When the core fiber is drawn polyethylene, the composite comprising the fibrous product preferably has a compressive strength of at least 16 kpsi (110 MPa), more preferably at least 20 kpsi (138 MPa).

The fibrous product and the composite containing it preferably withstand larger compressive strains before compression failure than the unsheathed core fiber, increasing the effort required to cause compression failure. The strain at which a composite comprising a fibrous product having an aramid core or polybenzazole is compression-failed is preferably at least 10 percent, more preferably at least 15 percent, and most preferably at least 19 percent.

The following examples are illustrative and not limiting. All parts and percentages are by weight unless otherwise indicated.

In this example, the fiber and composite compressive strengths are ASTM D
It is a small scale adaptation of −3410−82, measured by a mini-complex assay which gives similar results to the ASTM test in our experiments. 10 bundles of parallel fibers or fibrous products
Tactix 123 epoxy resin and Tactix Harde in a 0:17 weight ratio
Impregnated with ner H31 hardener and stacked uniaxially in a Teflon coated mold. Fill the mold with the same ratio of the same epoxy resin and hardener. The epoxy resin is cured to give a mini composite. The mold provides a test site containing the bundle and the cured epoxy resin, the test site having a cross section of 0.062 "x 0.125" and an axial length of 0.19 ". The fiber bundle extends beyond each end of the test site to an epoxy tab located at each end of the test site.
The ends of the tabs are cut using a diamond saw parallel to each other in a plane and perpendicular to the test site. The specimen is mounted on an Instron test machine and compressed until failure occurs. Record the stress and strain for failure. Composite compressive strength is obtained by dividing the failure stress by the cross-sectional area of the test site.

Example 1-Resin-Free Aramid Core Wrapped in Aramid Fiber The packaging mechanism is mounted in a centrifuge case and driven by a centrifugal motor American Volkmann Model No. V
It is configured to have a packaging element from TS-05-0 Twister. The packaging mechanism has an Accutense clutch mechanism Model No. 250 manufactured by Textrol that tensions the packaging fibers before entering the packaging element.

Kevl having the denier (core denier) shown in Table 1.
A core comprising parallel fibers of ar49 aramid fibers is wrapped with Kevlar 49 fibers having the denier shown in Table 1 (wrap denier) to form a fibrous product having the total denier shown in Table 1. Packaging speed is 7000 laps / m
in and the packaging coverage is 100 percent.

The wrapped fibers are again impregnated with the epoxy resin and hardener as described above and tested for compressive strength. Table 1 shows the test results. "No. of bundle in composite" means the total number of impregnated fibrous products in the test site of each test strip. "Total test denier" means the total denier of the packaged fibrous product contained in the minicomposite. "Total core denier" means the total denier of the core fibers contained in the minicomposite. "Load-failure" means the compressive load on the minicomposite when compression failure occurs. "Strain-failure" means the compressive strain of the minicomposite when compression failure occurs. "Avg composite compressive strength" means the average compressive strength calculated for this portion of the mini-composite.

Example 2-Aramid core core impregnated with epoxy resin before packaging Tactix 123 epoxy resin and Tactix before packaging
The method of Example 1 is repeated, except that it is impregnated with a Hardener H31 hardener. Table 2 shows the results.

Example 3-Variable Packing Tension The method shown in Example 2, Sample 2 (E) is repeated, except for the following.

The clutch mechanism applied a specific DC voltage to the clutch and passed the packaging point (1) immediately after the line exited the clutch while the packaging device was operating and (2) while the packaging device was stationary. At this time, by reading the tension with a Checkline tensiometer, a scale is set to determine an appropriate tension in the packaging of the generated packaging fiber. The first measurement does not include friction from the device and is lower than the actual packaging tension. The second measurement involves friction from the device that is not in contact with the fibers when the packaging device is operating and is higher than the actual packaging tension. The actual packaging tension is between the two. The results are shown in Table 3 (A).

Table 3 (A) Voltage tension (g) 10.0 21 to 80 12.5 25 to 94 15.0 27 to 104 17.5 33 to 121 20.0 36 to 149 22.5 43 to 164 25.0 55 to 183 27.5 63 to 207 30.0 73 to 258 The packaging and testing of the 1140 denier aramid core is repeated as described in Table 3 (B).

Example 4-A stretched polyethylene core impregnated with epoxy resin prior to packaging The procedure of Example 2 is repeated using Spectra polyethylene fibers as the core and 66 denier single filament nylon or 10 strands 7 denier rice filament nylon as the wrapping fiber. The variables and results are shown in Table 4.

Example 5 1300 denier polybenzoxazole fiber core and
The procedure of Example 3 is repeated using 200 denier Kevlar 49 aramid fiber packaging. The packaging fiber clutch was set at 14 volts. The core tension is 140-158g. The average denier of the packaged fiber is 2680 denier.

A composite sample having 12 bundles of packaged fibers is produced. The core denier of the composite is 15,600 and the total denier of the packaged fibers is 32,300. 25 bundles unpacked 13
Comparative complex containing 00 denier PBO (total denier 32,500)
To manufacture. 35kpsi (240MPa) packaged PBO complex
With a compressive strength of 22% and a strain-failure of 22%. The unpackaged PBO composite has a compressive strength of 18 kpsi (125 MPa) and 5.7 percent strain-failure.

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Claims (8)

(57) [Claims] 1. An outer core comprising: (a) a core comprising one or more essentially parallel core fibers; (b) an outer package comprising one or more packaging fibers surrounding the core and covering at least 50 percent of the outer surface of the core; (C) comprising a curable resin that is flowable prior to curing and can be cured to a cured resin having a compression modulus of at least 50,000 psi; (1) the core has an average diameter of 0.8 mm or less; Wherein the compressive strength of the core fiber comprises no more than 30 percent of its tensile strength; (2) the wrapping fibers surround the core under a radial compression of less than 0.1 percent; and (3) the curable resin comprises A fibrous product characterized by impregnating both core fibers and packaging fibers. 2. The fibrous product according to claim 1, wherein said core fibers comprise a polybenzoazole polymer or an aramid polymer or drawn polyethylene fibers. 3. The fibrous product according to claim 1, wherein said curable resin is an epoxy resin or a polycyanate resin. 4. The fibrous product according to claim 1, wherein said packaging fibers have a tension between 21 g and 1000 g. 5. The fibrous product of claim 1, wherein said packaging fibers cover at least 90 percent of the surface of said core. 6. The following steps: (1) Packaging with a tension of 21 g to 1000 g around a core having an average diameter of 0.8 mm or less, including a large number of core fibers whose compression strength is 30% or less of its tensile strength. Wrapping the fibers to form a fibrous product; and (2) flowable and at least 50,000 psi prior to curing.
A method for producing a fibrous product, comprising: impregnating the fibrous product with a curable resin that can be cured into a cured resin having a compression elastic modulus, and impregnating both the core fiber and the packaging fiber with the curable resin. 7. The following steps: (3) laminating a fibrous product impregnated with a curable resin in a shape of a selected molded article; and (4) curing the curable resin to form a matrix composite. 7. The method of claim 6, comprising forming. 8. The core of claim 6, wherein the wrapping fiber is wrapped around the core under a radial compression of less than 0.1 percent.
The described method.