Carbon fiber random mat and carbon fiber composite material

Abstract

A random mat material including fiber bundles, said fiber bundles including fibers having an average fiber length of 5 to 100 mm, and having an average number N of fibers in the fiber bundle that satisfies: $\frac{1.5{10}^5}{D^2}<N<\frac{4.5{10}^5}{D^2}$
wherein D is the average diameter of fibers in the fiber bundle, expressed in micrometers, and the standard deviation SD.sub.N of the number of fibers in a fiber bundle satisfies:
1,000<SD.sub.N<6,000
wherein at an end of the fiber bundle, the number of the fibers in a fiber bundle becomes less from center to edge of the fiber bundle in a fiber direction.

Claims

1. A method of producing a random mat comprising fiber bundles, said fiber bundles comprising fibers, said method comprising: (i) cutting a fiber bundle at essentially constant intervals with a cutting roll having a rotational speed vc, wherein
100 RPM<vc<400 RPM, and a cutting edge has a diagonal angle with respect to the fibers direction; and (ii) reducing the size of the fiber bundle, thereby providing the random mat.

2. The method according to claim 1, wherein the fiber bundles comprise fibers having an average fiber length of 5 to 100 mm and has an average number N of fibers in the fiber bundle N that satisfies: $\frac{1.5{10}^5}{D^2}<N<\frac{4.5{10}^5}{D^2}$ wherein D is the average diameter of fibers in the fiber bundle, expressed in micrometers, and the standard deviation SD.sub.N of the number of fibers in a fiber bundle satisfies:
1,000<SD.sub.N<6,000 and at an end of the fiber bundle, the number of the fibers in a fiber bundle becomes less from center to edge of the fiber bundle in a fiber direction.

3. The method according to claim 1, wherein the fiber bundles satisfy:
2,000<SD.sub.N<6,000.

4. The method according to claim 1, wherein the size of the fiber bundle is reduced by a stretching roll that is placed a distance L of 5 to 100 mm from a cutting roll, said cutting roll rotating at a rotational speed vc, said stretching roll rotating at a rotational speed vr; wherein vr is larger than vc.

5. The method according to claim 4, wherein the ratio of vr/vc satisfies:
20<vr/vc<80.

6. The method according to claim 4, wherein the roll is cylindrically or conically shaped.

7. The method according to claim 1, wherein 12K or 24K carbon fiber bundles are cut at step (i).

8. The method according to claim 1, wherein the mat is coated with a thermoset or thermoplastic polymer film or powder.

9. A composite material comprising the random mat according to the method defined in claim 1.

10. A molded fiber-reinforced article, wherein said molded fiber-reinforced article comprises one or more random mats according to claim 1, said molded fiber-reinforced article comprises 10 to 65% by mass of the fiber bundle with respect to the total mass of said fiber-reinforced article.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic side view of an example of our process.

(2) FIG. 2 is a schematic of another side view of an example of our process.

(3) FIG. 3 is a schematic of the process with stretching rolls.

(4) FIG. 4 is a schematic of a comparative process without stretching rolls.

(5) FIGS. 5, 6 and 7 are schematic side sections of examples of fiber bundles, but having different shapes at their ends. FIGS. 5, 6 and 7 show that at fiber bundle ends the number of the fibers in the fiber bundle becomes less from center to edge of the fiber bundle in the fiber direction. In FIG. 5, the fiber bundle has beveled ends. Beveled ends are formed because of the cutting step of the process according to the invention. In FIGS. 6 and 7, fiber bundles may present different shapes at the ends because during processing random mat, the bundle with beveled ends can be deformed as schematically represented in FIGS. 6 and 7. Such shapes can be observed in the random mat material.

DETAILED DESCRIPTION

(6) Hereinafter, our random mats, articles, and methods will be explained in detail together with Examples and Comparative Examples.

(7) Through the method described above, the random mat can be obtained without expensive machinery and equipment. Traditional machinery and equipment can be used, comprising a: a feeder to run fiber bundle to a cutting roll, b: a cutting roll with blades embedded in a line on the roll, and c: a basket to gather the cut fiber bundles below the cutting roll.

(8) FIG. 1 is a schematic view an example of our process. Fiber tow 1 is fed to cutting blade 4 through guide 8, by driving roll 6 and nip roll 7, and cut into fiber bundles 5 with the cooperation of stretching rolls 2.

(9) FIG. 2 is another schematic view of an example of our process showing the cutting blade 4 on the cutting roll 3. A cutting blade 4 is attached on the cutting roll 3 with a cutting edge forming an angle between the rotation direction and blade, corresponding to cut angle to fiber direction on tow.

(10) FIG. 3 is another schematic view of the process showing the fiber tow cut into fiber bundles according to an example of our process. The edge of fiber tow has an angle because the cutting blade 4 has an angle attached on the cutting roll 3. The fiber tow 1 is cut by cutting blade 4, which starts cutting fiber tow 1 at the side edge 41, and the edge 12 cut by the cutting blade is drawn by the stretching rolls and is separated from the fiber tow 1, thereby forming a fiber bundle 5. The fiber bundle or fiber tow 1 is therefore separated into multiple fiber bundles 5 forming elements or segments made of parts of the fiber bundle or fiber tow 1. Thanks to the stretching rolls 2, the number of fiber in the fiber bundles 5 is in a range defined below:

(11) $\frac{1{10}^5}{D^2}<N<\frac{4.5{10}^5}{D^2}$ N: average number of fibers in the fiber bundle D: average diameter of fibers in the fiber bundle (micrometers; m)

(12) FIG. 4 is a schematic view of an example of a comparative process without stretching rolls. The edge of fiber tow 1 has a diagonal angle because the cutting blade 4 forms a diagonal angle on the cutting roller 3 as in FIG. 3. The fiber tow 1 is cut by cutting blade 4, which starts cutting fiber tow 1 at the side edge 41 as in FIG. 3. Such a process comprises no means of reducing the size of the fiber tow 1. More precisely without stretching rolls 2, the edge 52 cut by the cutting blade remains in place until the full width of the fiber tow 1 is cut by cutting blade 4 and forms a fiber bundle 5 wherein the number of fibers is essentially the same as that of fiber tow 1. Therefore, the number of fibers in the fiber bundle 5 is out of our range. Hence, it is not possible to obtain a random mat with high mechanical properties or low production costs.

(13) Next, Examples and Comparative Examples are explained.

(14) First, the properties and determination methods are explained, then Examples and Comparative Examples are detailed.

(15) (1) Method of Determining Average Number of Fiber Bundles N and Standard Deviation SD:

(16) A sample with a size of 10 mm100 mm was cut out from a carbon fiber composite material and, thereafter, the sample was heated in an electric furnace heated at 500 C. for about one hour to burn off organic substances such as the matrix resin. The mass of carbon fiber aggregates left after cool down to a room temperature was determined. Carbon fiber bundles were all extracted from the carbon fiber aggregates by tweezers. All extracted carbon fiber bundles were weighted using a balance capable of measuring up to a degree of 1/10,000 g. The weight Mn and the length Ln of each carbon fiber bundle was determined. After the determination, for each bundle, xn=Mnx4/D.sup.2//Ln/S were calculated, wherein D is a diameter of carbon fibers, S is the specific gravity of carbon fibers, and xn is a number of fibers forming a carbon fiber bundle. 100 fiber bundles were picked up from the cut out materials and average bundle number N and standard deviation of SD were calculated from them.

(17) Mechanical Properties

(18) (2) Flexural Modulus

(19) Flexural modulus was determined according to ISO-14125.

(20) (3) Flexural Strength

(21) The flexural strength was determined according to ISO-14125.

EXAMPLES

Example 1

(22) A commercial fiber tow was selected (T700SC-12K-50C; Toray Carbon Fibers Europe, S. A.) and was set to the creel. The fiber diameter D was of 7 micrometers (m). The fiber tow was drawn to stretching rolls 2 via cutting rolls 3, nip roll 7, and driving roll 6 through a guide 8 (see FIG. 1). The distance between the stretching rolls 2 and cutting roll 3 was set to L=33 mm. A cutting blade 4 was set on the cutting roll 3 at a diagonal angle of 46 degree. Also, a resin film coated by commercial epoxy resin with 250 micrometers (m) thickness on a releasing paper was prepared just below the stretching rolls 2 to collect cut fiber bundles 5, which would form a random mat of fibers on the film after cutting. Then the stretching rolls 2 were started rotating at vr=9778 RPM and the cutting roll 3 was started rotating at vc=275 RPM. The resin film (not shown on FIG. 1) also started to feed at a speed of 10 mm/min to obtain a random mat on resin sheet surface. The resin sheet comprising the random mat was then cut into 30 cm30 cm square pieces and laid up to 10 layers. Then, the layers were set to a press molding machine and cured at 120 degree Celsius with 3 atmosphere pressure for 1 hour to obtain a composite material panel. Then, the panel was cut into coupon and the flexural modulus and flexural strength were evaluated in accordance with ISO 14125. The process parameters are shown in Table 1 and the results are shown in Table 2. The flexural modulus and strength are high enough to provide high mechanical properties composite materials and production costs were low thanks to the use of low cost 12K fiber tow.

(23) The width of fiber bundles became thinner from center to edge by decreasing number of fibers. The average number N of fiber bundles and standard deviation SD.sub.N were measured in accordance with the methods descried above, and are shown in Table 2.

Examples 2 to 6

(24) Fiber tow, rotation speed of cutting roll and stretching rolls were changed and the same evaluation as in example 1 was conducted. The process parameters and the results are shown in Table 1 and Table 2.

(25) The flexural modulus and flexural strength were high enough because average number N of fibers in fiber bundles were within the range of the present invention, and production costs were low thanks to the use of low cost 12K and 24K fiber tow.

Comparative Examples 1 to 3

(26) The conditions were the same as those in Example 1 except for fiber tow which was changed to T300-3K-40B and T300-1K-40B (both are commercial productions from Toray Industries, Inc.) and the evaluation was conducted as shown in Table 3. Results are shown in Table 4. The mechanical properties of the composite according to Comparative Example 1 were lower than that of Examples 1 to 6 because N was out of range. The mechanical properties of the composite according to Comparative Examples 2 and 3 were as good as Examples 1-6, but the production costs were higher than our examples because of usage of 3K and 1K fiber tows which price are higher than that of 12K or 24K.

(27) TABLE-US-00001 TABLE 1 Materials and process conditions of Examples 1 to 6 Distance between Angle Cut roller Stretch roller ratio Cut/Stretch L Example Fiber Matrix [] speed vc [/s] speed vr [/s] vr/vc [mm] 1 T700S- epoxy 46 275 RPM 9778 RPM 36 33 12K 2 T700S- epoxy 46 180 RPM 9778 RPM 54 33 12K 3 T700S- epoxy 46 180 RPM 12222 RPM 68 33 12K 4 T700S- epoxy 46 275 RPM 7333 RPM 27 33 12K 5 T700S- epoxy 46 180 RPM 4889 RPM 27 33 12K 6 T700S- epoxy 46 180 RPM 4889 RPM 27 33 24K

(28) TABLE-US-00002 TABLE 2 Results of Examples 1 to 6 satisfies Example Average N [x1000] $\frac{1.5{10}^5}{D^2}<N<\frac{4.5{10}^5}{D^2}$ StDev [x1000] Flexural modulus [GPa] StDev [MPa] Flexural strength [MPa] StDev [MPa] Vf [%] cost 1 4.62 Yes 3.12 32.1 11.7 283 47.2 50 low 2 4.43 Yes 3.21 31.1 12.1 277 37.2 48 low 3 3.87 Yes 5.28 33.2 9.4 290 35.3 48 low 4 6.75 Yes 3.66 35.6 10.2 280 41.3 50 low 5 5.89 Yes 3.08 33.4 10 274 42 52 low 6 5.89 Yes 3.08 34.5 11.2 269 39.7 47 low

(29) TABLE-US-00003 TABLE 3 Materials and process conditions of Comparative Examples 1 to 3 Comparative Cut example Fiber Matrix Angle [] roller speed vc [/s] 1 T700S-12K epoxy 90 275 RPM 2 T300-3K epoxy 90 275 RPM 3 T300-1K epoxy 90 275 RPM

(30) TABLE-US-00004 TABLE 4 Results of Comparative Examples 1 to 3 satisfies Comparative Example Average N [x1000] $\frac{1.5{10}^5}{D^2}<N<\frac{4.5{10}^5}{D^2}$ StDev [x1000] Flexural modulus [GPa] StDev [GPa] Flexural strength [MPa] StDev [MPa] Vf [%] cost 1 12.00 No 0.79 23.8 6.4 205 80.3 50 low 2 2.97 No 0.80 33.1 12.1 279 42.2 50 high 3 0.98 No 0.31 36.4 10.3 285 39.4 50 very high