Structure

Abstract

Provided is a structure having excellent flexibility represented by elastic restoring from compression or tensile elongation at break, and excellent lightness. A structure according to the present invention includes reinforced fibers, first plastic, and second plastic that exhibits rubber elasticity at room temperature, the reinforced fibers being discontinuous fibers, and the first plastic and/or the second plastic coating a crossing point between the reinforced fibers in contact with each other.

Claims

1. A structure of fiber-reinforced plastic comprising: reinforced fibers that are discontinuous fibers and have a mass average fiber length of 1 to 15 mm, a first plastic that exhibits no rubber elasticity at room temperature and has a softening point or melting point of 50° C. or higher, and a second plastic that exhibits rubber elasticity at room temperature, has a tensile elongation at break of 200% or more, and has a tensile strength at break of 10 MPa or more, wherein crossing points between the reinforced fibers in contact with each other have a first inner coating of the first plastic and a second outer coating of the second plastic, and the structure of fiber-reinforced plastic further comprises voids as spaces formed by the first plastic and second plastic-coated reinforced fibers that are columnar supporting bodies and overlap or cross with each other, wherein a volume content of the voids in the structure is in a range of 10 vol % or more and 97 vol % or less.

2. The structure according to claim 1, wherein the structure has a density of 0.01 g/cm.sup.3 or more and 1.3 g/cm.sup.3 or less.

3. The structure according to claim 1, having an elastic restoring from 50% compression of 1 MPa or more.

4. The structure according to claim 1, having a tensile elongation at break in a range of 1% or more and 20% or less.

5. The structure according to claim 1, wherein the reinforced fibers have a tensile elongation at break in a range of 1% or more and 10% or less.

6. The structure according to claim 1, wherein the reinforced fibers contain at least one selected from the group consisting of PAN-based carbon fibers, PITCH-based carbon fibers, glass fibers, and aramid fibers.

7. The structure according to claim 1, wherein the first plastic and/or the second plastic coating the crossing point between the reinforced fibers has a coating thickness in a range of 1 μm or more and 15 μm or less.

8. The structure according to claim 1, wherein the second plastic contains at least one selected from the group consisting of silicone rubber, ethylene propylene rubber, acrylonitrile butadiene rubber, chloroprene rubber, fluororubber, a polyolefin-based thermoplastic elastomer, a polyurethane-based thermoplastic elastomer, a polyester-based thermoplastic elastomer, and a polyamide-based thermoplastic elastomer.

9. The structure according to claim 1, wherein the first plastic is 5 parts by mass or more and 25 parts by mass or less relative to 100 parts by mass of the reinforced fibers.

Description

EXAMPLES

(1) Hereinafter, the present invention is further specifically described by way of examples.

(2) (1) Volume Content of Voids in Structure

(3) A 10-mm square test piece was cut out from the structure, a section of the test piece was observed with a scanning electron microscope (SEM) (S-4800 type, manufactured by Hitachi High-Technologies Corporation), and the surface of the structure was imaged at 10 locations with an equal interval at a magnification of 1000 times. A void area A.sub.a in each of the images was obtained. A percentage of the voids was calculated by dividing the void area A.sub.a by the area of the entire image. The volume content of the voids in the structure was obtained by an arithmetic average of percentages of the voids at a total of 50 locations, 10 locations in each of 5 test pieces.

(4) (2) Density of Structure

(5) A test piece was cut out from the structure and the apparent density of the structure was measured with reference to JIS K7222 (2005). The dimension of the test piece was 100-mm square. The length, width, and the thickness of each of the test pieces were measured by a micrometer, and a volume V of the test piece was calculated from the obtained values. In addition, a mass M of the cut-out test piece was measured by an electronic balance. The obtained mass M and volume V were substituted in the following formula to calculate a density p of the structure.
ρ [g/cm.sup.3]=10.sup.3×M [g]/V [mm.sup.3]

(6) (3) Elastic Restoring from 50% Compression of Structure

(7) A test piece was cut out from the structure and the compression property of the structure was measured with reference to JIS K7220 (2006). The test piece was cut out at a length of 25±1 mm and a width of 25±1 mm. The obtained test piece was measured for its compression property using a universal tester. In the measurement, a compression strength σ.sub.m was calculated by the following formula using a maximum force F.sub.m at a deformation rate of 50% and a bottom sectional area A.sub.0 of the test piece before the test, and the calculated value was defined as the elastic restoring. Used as a measuring device was an “INSTRON (registered trademark)” 5565 type universal material testing machine (manufactured by INSTRON JAPAN Co., Ltd.).
σ.sub.m [MPa]=F.sub.m [N]/A.sub.0 [mm.sup.2]

(8) (4) Tensile Elongation at Break of Structure

(9) A test piece was cut out from the structure and the tensile property of the structure was measured with reference to JIS K6400 (2012). The test piece was cut out in the No. 1 shape. The obtained test piece was measured for its tensile property using a universal tester. Used as a measuring device was an “INSTRON (registered trademark)” 5565 type universal material testing machine (manufactured by INSTRON JAPAN Co., Ltd.).

(10) (5) Coating Thickness of Plastic in Structure

(11) The structure was cut out into a 10-mm square test piece, a section of the test piece was observed with a scanning electron microscope (SEM) (S-4800 type manufactured by Hitachi High-Technologies Corporation), and any 10 locations were imaged at a magnification of 3000 times. The coating thickness of the plastic coating crossing points of the reinforced fibers was measured at any 50 locations showing cut sections of the crossing points of the reinforced fibers in the obtained images, and an arithmetic average of the 50 locations is defined as the coating thickness of the plastic.

(12) (6) Content of First Plastic Relative to Reinforced Fibers

(13) The reinforced fibers that had not been coated with the first plastic were cut at a length of 25±1 mm and a width of 25±1 mm and measured for their mass W1. Thereafter, the reinforced fibers having the first plastic contained therein were measured for their mass W2. A content of the first plastic Wr was calculated by the following formula and the amount of the first plastic per 100 parts by mass of the reinforced fibers was calculated.
Content of first plastic relative to reinforced fibers Wr (mass)=W2−W1

(14) (7) Softening Point or Melting Point of First Plastic

(15) The melting point was evaluated by a differential scanning calorimeter (DSC). A sample in an amount of 5 mg was placed in a sealing sample container, the temperature was raised from 30° C. to 300° C. at a temperature rise rate of 10° C./min, and the sample was evaluated. As an evaluation device, PyrisIDSC manufactured by PerkinElmer was used.

(16) When it was difficult to evaluate the first plastic by the melting point (when the first plastic had no melting point), the first plastic was evaluated for its Vicat softening temperature in accordance with ISO306 (2004) (using a force of 10 N), and the evaluation result was defined as the softening point.

(17) (8) Tensile Property of Second Plastic

(18) The second plastic was subjected to a tensile test performed with reference to a method described in JIS K6400 (2012) and evaluated for its tensile elongation at break and tensile strength at break. The obtained test piece was measured for its tensile property using a universal tester. Used as a measuring device was an “INSTRON (registered trademark)” 5565 type universal material testing machine (manufactured by INSTRON JAPAN Co., Ltd.).

(19) The second plastic was determined to be rubber elastic or anelastic by the test performed by releasing a stress at 200% extension (with the length of the test piece defined as the standard 100%) and confirming by visual inspection whether the second plastic returned its shape to 150% or less. When returning its shape to 150% or less, the second plastic was determined to be “rubber elastic,” whereas when returning its shape to more than 150% or broken, the second plastic was determined to be “anelastic.”

(20) The test piece was produced in the No. 1 dumbbell-shaped test piece shape and subjected to the test. As regards the second plastic that was thermoplastic, the test piece was produced by injection molding. As regards the second plastic that exhibited liquid property at room temperature, the test piece was produced by casting the second plastic into a mold having a recess with the same shape as the No. 1 dumbbell-shaped test piece, closing the mold, and then curing the second plastic at a temperature/time for cross-linking or curing.

(21) (9) Volume Content of Reinforced Fibers in Structure Vf

(22) After a mass of the structure Ws was measured, the structure was heated in air at 500° C. for 30 minutes to burn off its plastic component, a mass of the remaining reinforced fibers Wf was measured, and the volume content of the reinforced fibers was calculated by the following formula. At this time, used as the densities of the reinforced fibers and the plastic were measurement results obtained by a weight measurement method in liquid in JIS Z8807 (2012).
Vf (vol %)=(Wf/ρf)/{Wf/ρf+(Ws−Wf)/ρr}×100
ρf: density of reinforced fibers (g/cm.sup.3)
ρr: density of plastic (g/cm.sup.3)

(23) (10) Volume Content of First Plastic

(24) A structure precursor only formed of the reinforced fibers and the first plastic was produced, and the volume content of the first plastic was obtained by the following formula using a value of the volume content of the voids in the precursor obtained in the same manner as in (1) and using a value of the volume content of the reinforced fibers.
Vr1 of first plastic (vol %)=100−(Vf+Va)
Vf: volume content of reinforced fibers (vol %)
Va: volume content of voids (vol %)
Vr1: volume content of first plastic (vol %)

(25) (11) Volume Content of Second Plastic

(26) The volume content of the second plastic was obtained by the following formula using values of the volume content of the voids, the volume content of the reinforced fibers, and the volume content of the first plastic in the structure that were obtained in (1), (9), and (10).
Vr2 of second plastic (vol %)=100−(Vf+Va+Vr1)
Vf: volume content of reinforced fibers (vol %)
Va: volume content of voids (vol %)
Vr2: volume content of second plastic (vol %)

(27) (12) Plastic Coat Proportion in Structure

(28) The structure was cut out into a 10-mm square test piece, a section of the test piece was observed with a scanning electron microscope (SEM) (S-4800 type manufactured by Hitachi High-Technologies Corporation), and any 10 locations were imaged at a magnification of 1000 times. As regards crossing points of the reinforced fibers, any 40 locations of the obtained images were measured for the number of crossing points of the reinforced fibers and the number of plastic-coated locations among the crossing points of the reinforced fibers, and a value obtained by the following formula was defined as the plastic coat proportion (%).
Plastic coat proportion (%)=(C2/C1)×100
C1: number of crossing points measured (pieces)
C2: number of crossing points coated with plastic among C1 (pieces)

(29) The following materials were used in the following examples and comparative examples.

(30) [Carbon Fibers]

(31) A copolymer containing polyacrylonitrile as a main component was subjected to spun processing, calcined processing, and surface oxidation treatment processing, and a total of 12,000 single yarns were obtained as continuous carbon fibers. The properties of the continuous carbon fibers were as follows.

(32) Specific gravity: 1.8

(33) Tensile strength: 4600 MPa

(34) Tensile elastic modulus: 220 GPa

(35) Tensile elongation at break: 2.1%

(36) [Aramid Fibers]

(37) Aramid fibers (“Kevlar” (registered trademark) 29 manufactured by DU PONT-TORAY CO., LTD.)

(38) Specific gravity: 1.44

(39) Tensile strength: 2900 MPa

(40) Tensile elastic modulus: 70 GPa

(41) Tensile elongation at break: 3.6%

(42) [Polyamide]

(43) As the first plastic, water-soluble polyamide plastic (“AQ nylon” (registered trademark) P-70 from Toray Industries, Inc.) was used.

(44) Softening point: 85° C.

(45) [Polyurethane]

(46) As the first plastic, a polyurethane water dispersion (“SUPERFLEX” (registered trademark) 150 from DKS Co., Ltd.) was used.

(47) Softening point: 195° C.

(48) Melting point: 212° C.

(49) [Polyester Plastic]

(50) A plastic film was produced that was formed of polyester plastic (“Hytrel” (registered trademark) SB754 manufactured by Toray Industries, Inc.) with a weight per unit area of 121 g/m.sup.2, and the plastic film was used as the second plastic. Table 1 shows the properties of the obtained plastic film.

(51) [Silicone Rubber]

(52) Silicone rubber (RBL-9200-40 manufactured by Dow Toray Co., Ltd.) was used. An A agent (main agent) and a B agent (curing agent) of the silicone rubber were mixed at a mixing ratio of 1:1, an amount of 124 g/m.sup.2 was extracted and stirred to produce the silicone rubber, which was used as the second plastic. Table 1 shows the properties of the silicone rubber.

(53) [Epoxy Plastic]

(54) An uncured epoxy plastic composition was prepared by heating and kneading, with a kneader, epoxy plastic (30 parts by mass of “EPIKOTE” (registered trademark) 828, 35 parts by mass of “EPIKOTE” (registered trademark) 1001, and 35 parts by mass of “EPIKOTE” (registered trademark) 154 from Japan Epoxy Resins Co., Ltd.) and 5 parts by mass of thermoplastic polyvinyl formal (“Vinylec” (registered trademark) K from CHISSO CORPORATION) to uniformly dissolve polyvinyl formal, and then kneading, with a kneader, 3.5 parts by mass of a curing agent dicyandiamide (DICY7 from Japan Epoxy Resins Co., Ltd.) and 7 parts by mass of a cure accelerator 4,4-methylenebis(phenyldimethylurea) (“OMICURE” (registered trademark) 52 from PTI Japan Limited). A plastic film with a weight per unit area of 132 g/m.sup.2 was produced from the uncured epoxy plastic composition with a knife coater, and the plastic film was used as the second plastic. Table 1 shows the properties of the obtained plastic film.

Example 1

(55) With use of the carbon fibers as the reinforced fibers, the carbon fibers were cut with a strand cutter at 6 mm to give chopped carbon fibers. A dispersion liquid formed of water and a surfactant (polyoxyethylene lauryl ether (trade name) manufactured by NACALAI TESQUE, INC.) at a concentration of 0.1 mass % was prepared, and a fiber-reinforced mat was manufactured with use of the dispersion liquid and the chopped carbon fibers. A manufacturing device includes, as a dispersing tank, a 1000-mm diameter cylinder-shaped container having an opening cock at a lower portion of the container, and includes a linear transport part (inclination angle: 30°) connecting the dispersing tank to a paper-making tank. A stirrer is attached to an opening on an upper surface of the dispersing tank, and it is possible to charge the chopped carbon fibers and the dispersion liquid (dispersion medium) through the opening into the dispersing tank. The paper-making tank includes a mesh conveyor having a 500-mm wide paper-making surface at the bottom and has the mesh conveyor thereof connected to a conveyor capable of delivering a carbon fiber substrate (paper-making substrate). Paper making was performed with the concentration of the carbon fibers in the dispersion liquid set at 0.05 mass %. The fiber-reinforced mat produced by the paper making was dried in a dry furnace at 200° C. for 30 minutes to give a fiber-reinforced mat. The weight per unit area of the mat was 50 g/m.sup.2.

(56) The polyamide as the first plastic was dissolved in water to give a concentration of 1 mass %. The aqueous polyamide solution was applied to the fiber-reinforced mat obtained above. The fiber-reinforced mat having the aqueous polyamide solution applied thereto was put in a hot air oven whose temperature was adjusted to 110° C. and dried for 2 hours to give a first structure precursor. The attachment rate of the polyamide to the obtained first structure precursor was 10 parts by mass relative to 100 parts by mass of the fiber-reinforced mat.

(57) A laminated product was produced by disposing the polyester plastic as the second plastic on the first structure precursor in an order of [second plastic/first structure precursor/second plastic/first structure precursor/second plastic/first structure precursor/second plastic/first structure precursor/first structure precursor/second plastic/first structure precursor/second plastic/first structure precursor/second plastic/first structure precursor/second plastic]. Next, the laminated product was subjected to the following steps (1) to (5) to give a structure. Table 2 shows the properties of the structure.

(58) (1) The laminated product is disposed in a press-molding mold cavity that has been preliminarily heated at 200° C. and the molds are closed.

(59) (2) Next, the molds are retained for 120 seconds and then further retained for 60 seconds while pressed at a pressure of 3 MPa.

(60) (3) After the step (2), the mold cavity is opened and a metal spacer is inserted at an end of the cavity to adjust the thickness of an obtained structure to 3.4 mm.

(61) (4) Thereafter, the mold cavity is closely closed again, and the molds are cooled to a cavity temperature of 50° C. while the pressure is retained.

(62) (5) The molds are opened and a structure is extracted.

Example 2

(63) A laminated product was obtained that was the same as that of Example 1 except for having a content of the first plastic of 8 parts by mass, and next, the laminated product was subjected to the following steps (1) to (4) to give a structure. Table 2 shows the properties of the structure.

(64) (1) The laminated product is disposed in a press-molding mold cavity that has been preliminarily heated at 200° C. and the molds are closed.

(65) (2) Next, the molds are further retained for 120 seconds while pressed at a pressure of 3 MPa.

(66) (3) Thereafter, the molds are cooled to a cavity temperature of 50° C. while the pressure is retained.

(67) (4) The molds are opened and a structure is extracted.

Example 3

(68) A laminated product was obtained that was the same as that of Example 1 except for containing as the first plastic the polyurethane instead of the polyamide, having a content of the first plastic of 10 parts by mass, and having a mass proportion of the reinforced fibers in the structure of 55 mass %, and next, the laminated product was subjected to the following steps (1) to (5) to give a structure. Table 2 shows the properties of the structure.

(69) (1) The laminated product is disposed in a press-molding mold cavity that has been preliminarily heated at 200° C. and the molds are closed.

(70) (2) Next, the molds are retained for 120 seconds and then further retained for 60 seconds while pressed at a pressure of 3 MPa.

(71) (3) After the step (2), the mold cavity is opened and a metal spacer is inserted at an end of the cavity to adjust the thickness of an obtained structure to 5.9 mm.

(72) (4) Thereafter, the mold cavity is closely closed again, and the molds are cooled to a cavity temperature of 50° C. while the pressure is retained.

(73) (5) The molds are opened and a structure is extracted.

Example 4

(74) In Example 4, the second plastic was changed from the polyester plastic to the silicone rubber. Eight pieces of the first structure precursors used in Example 1 were laminated and stored in a stainless steel container into which the silicone rubber was charged, and the first structure precursors were stroked with a hand roller until impregnated with the silicone rubber, to produce a laminated product. Next, the laminated product was subjected to the following steps (1) to (4) to give a structure. Table 2 shows the properties of the structure.

(75) (1) The laminated product is disposed in a press-molding mold cavity that has been preliminarily heated at 150° C. and the molds are closed.

(76) (2) Next, the molds are further retained for 60 minutes while pressed at a pressure of 3 MPa.

(77) (3) Thereafter, the molds are cooled to a cavity temperature of 30° C. while the pressure is retained.

(78) (4) The molds are opened and a structure is extracted.

Example 5

(79) A laminated product was obtained in the same manner as in Example 4. Next, the laminated product was subjected to the following steps (1) to (4) to give a structure. Table 2 shows the properties of the structure.

(80) (1) The laminated product is disposed in a press-molding mold cavity that has been preliminarily heated at 150° C.

(81) (2) Next, a metal spacer is inserted at an end of the mold cavity to adjust the thickness of an obtained structure to 3.3 mm, and the molds are closed then retained for 10 minutes.

(82) (3) Thereafter, the molds are cooled to a cavity temperature of 30° C. while the pressure is retained.

(83) (4) The molds are opened and a structure is extracted.

Example 6

(84) A first structure precursor was obtained in the same manner as in Example 1. A structure was obtained in the same manner as in Example 1 except that the laminated product was obtained by adjusting the amount of the second plastic to give a mass proportion of the reinforced fibers of 55 mass % in the structure. Table 2 shows the properties of the structure.

Example 7

(85) A structure was obtained in the same manner as in Example 1 except that the reinforced fibers were changed from the carbon fibers to the aramid fibers, the first plastic was changed from the polyamide to the polyurethane, and the mass proportion of the aramid fibers in the structure was changed to 25 mass %. Table 2 shows the properties of the structure.

Example 8

(86) A structure was obtained in the same manner as in Example 2 except that the reinforced fibers were changed from the carbon fibers to the aramid fibers, the first plastic was changed from the polyamide to the polyurethane, the content of the first plastic was changed to 10 parts by mass, and the mass proportion of the aramid fibers in the structure was changed to 25 mass %. Table 2 shows the properties of the structure.

Comparative Example 1

(87) A laminated product was obtained that was the same as that of Example 1 except for having a content of the first plastic of 8 parts by mass and containing as the second plastic the epoxy plastic instead of the polyester plastic, and next, the laminated product was subjected to the following steps (1) to (4) to give a structure. Table 3 shows the properties of the laminate.

(88) (1) The laminated product is disposed in a press-molding mold cavity that has been preliminarily heated at 150° C. and the molds are closed.

(89) (2) Next, the molds are further retained for 10 minutes while pressed at a pressure of 3 MPa.

(90) (3) Thereafter, the molds are cooled to a cavity temperature of 50° C. while the pressure is retained.

(91) (4) The molds are opened and a structure is extracted.

Comparative Example 2

(92) A laminated product was formed by changing the first plastic from the polyamide to the polyurethane and adjusting the content of the first plastic to 30 parts by mass relative to the reinforced fibers. Next, the laminated product was subjected to the following steps (1) to (4) to give a structure. Table 3 shows the properties of the laminate.

(93) (1) The laminated product is disposed in a press-molding mold cavity that has been preliminarily heated at 150° C.

(94) (2) Next, a metal spacer is inserted at an end of the mold cavity to adjust the thickness of an obtained structure to 1.5 mm, and the molds are closed then retained for 10 minutes.

(95) (3) Thereafter, the molds are cooled to a cavity temperature of 50° C. while the pressure is retained.

(96) (4) The molds are opened and a structure is extracted.

Comparative Example 3

(97) A laminated product was produced by changing the reinforced fibers from the carbon fibers to the aramid fibers, using no first plastic, and using the epoxy plastic as the second plastic. Next, the laminated product was subjected to the following steps (1) to (4) to give a structure. Table 3 shows the properties of the laminate.

(98) (1) The laminated product is disposed in a press-molding mold cavity that has been preliminarily heated at 150° C. and the molds are closed.

(99) (2) Next, the molds are further retained for 10 minutes while pressed at a pressure of 3 MPa.

(100) (3) Thereafter, the molds are cooled to a cavity temperature of 50° C. while the pressure is retained.

(101) (4) The molds are opened and a structure is extracted.

(102) [Study]

(103) Examples 1, 3, and 5 to 7 gave a result that as regards the structure containing the reinforced fibers, the first plastic, and the second plastic that exhibits rubber elasticity at room temperature, when the structure contains discontinuous fibers as the reinforced fibers and the second plastic coats the crossing point between reinforced fibers that is bound by the first plastic, the structure has an elastic restoring from 50% compression of 1 MPa or more and a tensile elongation of 1% or more in any cases. Also in Examples 2, 4, and 8 in which the elastic restoring from 50% compression was unmeasurable due to a small percentage of voids, their structures were clarified to have excellent tensile elongation at break in comparison with the structures of Comparative Examples 1 and 3 and clarified to have both excellent flexibility and lightness. Comparison of Examples 1 to 8 with Comparative Example 3 affirmed that coating the reinforced fibers with the first plastic prevents the mat formed of the reinforced fibers from going to pieces during conveyance and makes the mat excellent in handleability. In Comparative Examples 1 and 3, the structure contained as the second plastic the epoxy plastic that had no rubber elasticity at room temperature, so that the structure never exhibited elastic restoring from 50% compression. The structure of Comparative Example 1 had a tensile elongation of 1% or more. The structure, however, had no voids, so that such a tensile elongation was considered to be a reflection of the tensile elongation of the reinforced fibers. Examples 3 and 7 were capable of giving structures having an appropriate tensile elongation at break even though the type of the reinforced fibers was changed. The above results make it clear that the structure in the scope of the present invention has an excellent compression property and an excellent tensile property.

(104) TABLE-US-00001 TABLE 1 Polyester Silicone Epoxy Type — plastic rubber plastic Density g/cm.sup.3 1.09 1.13 1.20 Melting point ° C. 160 — — Softening point ° C. 55 — — Rubber elasticity Rubber Rubber Rubber Anelastic elastic or elastic elastic anelastic Tensile elongation % 900 580 10 at break Tensile strength MPa 10 10 60 at break

(105) TABLE-US-00002 TABLE 2 Example 1 Example 2 Example 3 Example 4 Reinforced fibers Type Carbon fibers Carbon fibers Carbon fibers Carbon fibers Tensile elongation % 2.1 2.1 2.1 2.1 at break First plastic Type Polyamide Polyamide Polyurethane Polyamide Softening point ° C. 85 85 195 85 Melting point ° C. — — 212 — Amount relative to Parts by 10 8 10 8 100 parts by mass of mass reinforced fibers Second plastic Type Polyester Polyester Polyester Silicone plastic plastic plastic rubber Coating of crossing Present Present Present Present Present point between or reinforced fibers absent Coating thickness of μm 4.8 4.8 2.0 4.8 first plastic and second plastic Structure — Density g/cm.sup.3 0.40 1.23 0.23 1.27 Volume content of Vol % 66.7 0 83.3 0 voids Volume content of Vol % 6.8 20.4 7.3 21.2 total of reinforced fibers and first plastic Volume content of Vol % 26.5 79.6 9.4 78.8 second plastic Elastic restoring MPa 8.0 Unmeasurable 3.0 Unmeasurable (from 50% compression) Tensile elongation % 13.0 3.0 8.0 3.0 at break Example 5 Example 6 Example 7 Example 8 Reinforced fibers Type Carbon fibers Carbon fibers Aramid fibers Aramid fibers Tensile elongation % 2.1 2.1 4.4 4.4 at break First plastic Type Polyamide Polyamide Polyurethane Polyurethane Softening point ° C. 85 85 195 195 Melting point ° C. 212 — 212 212 Amount relative to Parts by 10 10 10 10 100 parts by mass of mass reinforced fibers Second plastic Type Silicone Polyester Polyester Polyester rubber plastic plastic plastic Coating of crossing Present Present Present Present Present point between or reinforced fibers absent Coating thickness of μm 4.8 2.0 5.0 5.0 first plastic and second plastic Structure — Density g/cm.sup.3 0.42 0.23 0.39 1.16 Volume content of Vol % 66.7 83.3 66.7 0 voids Volume content of Vol % 7.1 7.1 6.7 20.1 total of reinforced fibers and first plastic Volume content of Vol % 26.2 9.6 26.6 79.9 second plastic Elastic restoring (from 50% MPa 5.0 3.0 3.0 Unmeasurable compression) Tensile elongation % 12.0 5.0 19.0 4.0 at break

(106) TABLE-US-00003 TABLE 3 Comparative Comparative Comparative example 1 example 2 example 3 Reinforced fibers Type Carbon fibers Carbon fibers Aramid fibers Tensile elongation % 2.1 2.1 4.4 at break First plastic Type Polyamide Polyurethane None Softening point ° C. 85 195 — Melting point ° C. — 212 — Amount relative to 100 Parts by 8 30 — parts by mass of mass reinforced fibers Second plastic Type Epoxy plastic None Epoxy plastic Coating of crossing point Present or Present — Present between reinforced fibers absent Volume content of first plastic μm 4.8 0.9 1.9 or second plastic Structure — Density g/cm.sup.3 1.32 0.74 1.31 Volume content of voids Vol % 0 33.3 0 Volume content of total of Vol % 20.2 18.2 45.5 reinforced fibers and first plastic Volume content of first Vol % 79.8 48.4 54.5 plastic or second plastic Elastic restoring MPa Unmeasurable 0.3 Unmeasurable (from 50% compression) Tensile elongation at % 2.0 0.2 0.8 break

INDUSTRIAL APPLICABILITY

(107) According to the present invention, it is possible to provide a structure having excellent flexibility represented by elastic restoring from compression or tensile elongation at break, and excellent lightness.