DE2122236A1 - Carbon fiber reinforced graphite articles and process for their manufacture - Google Patents

Description

DR. BERG DIPL.-ING. STAPF

PATENTANWÄLTE ■ MÜNCHEN SO, MAUKRKIRCHERSTR. 48

Dr. If W »U» B. »Frf. · MOwdi— W, MmMrUrdMntrae «45 Your Sdiralb · «

20 990 ο »·«. May 5th, 1971

Lawyer file no. 20 990 '

United Kingdom Atomic Energy Authority 11, Charles II Street, London SV / 1 / England

"Carbon Fiber Reinforced Graphite Articles and Processes for Their Manufacture"

The invention relates to carbon fiber reinforced carbon structure and allows graphite articles of high modulus and high strength, which also has a high them! see gap resistance and low density.

10 $ t47 / m0 -2-

Godfather 24/9918/10

The invention provides a method for producing or further processing a carbon fiber-reinforced graphite article which comprises impregnating a bundle of aligned carbon fibers with a liquid containing a thermosetting resin, the liquid having a viscosity that increases with time and temperature, heating the impregnated bundle to convert the ψ liquid contained therein into a viscous mass, whereby the fibers are held together, deforming the fiber bundle obtained, heating the deformed fiber bundle to hard-harden the resin and to solidify the viscous mass located between the fibers, heating the resin-hardened bundle in a non-oxidizing atmosphere for carbonizing (charring) the solidified mass and then heating the carbonized and .deformed bundle in a non-oxidizing atmosphere and at a higher temperature to graphitize the carbonized material.

Before the resin is impregnated, the carbon fibers of the bundle can be surface-oxidized. The fibers may preferably be surface-oxidized by heating in air at 400 to 600 ⁰ C by heating in air at an elevated temperature. Alternatively, the fibers can be surface-oxidized by immersion in a solution of an oxidizing agent. The solution can be an aqueous solution of nitric acid or

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Be sodium liypochlorite, the latter being preferred, i.s viird an ^ enoirx.en that the bond at the fiber-resin interface is significantly improved if the carbon fibers receive a preparatory oxidation treatment.

As an alternative to, or in connection with, surface oxidation, the fibers can be surface treated by immersion in a solution of a compound containing a carbide-forming element. Suitable carbide-forming elements include beryllium, Eor, Matriuu and silicon and suitable compounds include NaCl, Η, ΒΟ, IJa ₂ SiO, and Be (^ 0,) _. It is believed that this latter treatment of applying a carbide-forming element to the fiber surfaces enables the element to intercalate with the carbon atoms of the fibers and the matrix, or by forming a carbide bridge, thereby providing improved bonding between the fibers Fibers and the carbonized or graphitized material is produced. As is well known in the coal field, intercalation is a process whereby ions are absorbed by carbon; as a result, the carbon structure is stretched and the resulting increase in the surface area leads to a better connection in the case of the fibers. The fiber bundle can be deformed by applying pressure to it , for example by compression molding. It is preferred

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BATH ORIGINAL

the mold pressure during heating for hard curing of the resin and the solidification of the viscous mass on it.

The thermosetting resin can itself be a liquid which can be used for impregnating the fiber bundle: optionally the liquid can be a solution or

^ be a resin powder in a volatile solvent. Suitable resins include a phenol-hesol resin, an epoxy resin, a furan, a xylenol-formaldehyde resin, a phenol novolak resin, a polyphenylene resin, a polynaphthalene resin, a coal tar pitch or a mixture of two or more of these substances can be selected. Suitable solvents for solid resin powders include ethyl alcohol, acetone, toluene and xylene. Phenol-resole resins and polyphenylene resins provide graphite articles that have the highest modulus and strength values and are accordingly

P preferred.

After the carbonizing heat treatment, shrinkage and porosity appear in the carbonized mass and it is preferred to impregnate the carbonized mass with further liquid in a vacuum and to subject the impregnated carbonized bundle to pressure, with subsequent solidification and carbonization the so one

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BATH ORIGINAL

guided impregnated liquid. The liquid impregnation, the solidification and the carbonizing heat treatment can be repeated as necessary in order to eliminate voids in the carbonized material before the graphitization.

After the resin has been hard-cured , post-cure by heat-treating the formed fiber bundles in air is preferred in order to improve the yield of carbon and promote dimensional stability during carbonization. A suitable temperature for post-hardening is ⁰

In order to increase the flexural strength of the graphite articles, which according to the. present invention can be made to increase « Carbon black or graphite can be dispersed in the uncured resin as a powder.

The invention also provides carbon fiber-reinforced graphite * articles which, as mentioned above, are processed further will.

The essence of the present invention will be further developed Examples are explained with reference to the following embodiments.

BATH ORIGINAL

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Carbon fiber bundle comprising 10,000 fibers per bundle cross-section were prepared by immersion in an aqueous solution of Natriunihypochlorxt (19 wt -.% Available chlorine containing) surface behändeIt. Instead of the hypochlorite solution, a 60 to 50 vol .- ^ aqueous nitric acid solution can be used, but the use of a

Hypochlorite solution preferred.

The bundles are then reacted with a phenolic resole resin such as Resin RIO993 supplied by BXL Ltd., impregnated. The fiber bundles are passed through a bath of the liquid resin under tension. The viscosity of the liquid resin can be adjusted appropriately by using a suitable solvent, for example acetone; Water or ethyl alcohol can also be used with phenolic resins. The viscosity de · resin, for example, a control value of 50 centipoises at about 20 ⁰ C determines the amount of resin on * would take in the fibers wherein the objective is "a controlled resin content in the to obtain fiber-blend resin . Fiber-resin mixtures containing from 30 to 70 % by weight of fibers were produced. The influence of the fiber content is discussed further below.

The resin impregnated bundles then became partial at a temperature in the range of 100 to 20O ° C

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BATH ORIGINAL

Hardening of the resin is heated and as a result a viscous mass of B-stage resin located between the carbon fibers is formed and this viscous mass holds the fibers together.

The fiber bundles are then deformed by compression molding. Cut Bundle lengths are placed in a mold recess, with the direction-dependent properties of the final Article can be determined by the chosen orientation (s) of the fibers in the shape: the shape is then closed and pressure applied to deform the fibers into the desired shape.

Simultaneously with the applied pressure, the fiber bundle pressed parts are heated to harden the viscous resin and to form a hard-hardened resin between the fibers. A suitable compression pressure has been found to be in the range of 0.5 to 1.0 tons / eq inch (0.77 ¹ I-IfS ¹ IB kp / mm ² ). The Härtungetemperatur was 150 ⁰ C. To the curing Hitrebehandlung post curing was joined in air at 200 ° C for 1 hour.

The post-hardened pressed parts were then placed in an inert atmosphere of argon to carbonize the hardened Harsee. heated. This was achieved with a tea temperature increase of 10 ° C / 3td. causes and a 1 hour dwell at 980 bit 1020 ⁰ C.

ORIGIMAL BATHROOM

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To compensate for the shrinkage and porosity caused by the carbonized heat treatment, further phenolic resin was introduced.

Each carbonized pressed part was placed in a vacuum chamber, the pressure reduced and the pressed part solidified at <1 mm Hg in 1 to 3 hours. The pressing was anschlies- W send completely submerged in a further Menjse of the liquid resin at the same pressure and stored in the chamber. The excess pressure was then released and 100 psi (7.0 kg / cm) applied to the vacuum-impregnated pressed part in order to solidify the structure in 1 to 3 hours.

After each impregnation, the pressed part was first reheated in air to harden it and then in argon to carbonize the freshly introduced resin . Finally, the pressed parts were subjected to a high-temperature heat treatment in argon in order to graphitize the carbonized material; A suitable graphitization temperature is 260O ⁰ C, and the pressed parts were kept at this temperature for 1 to 4 hours, generally 2 hours.

The flexural strength of the earbooised pressed parts produced as described above with, for example, “Sum.

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Content of 60% by weight of unidirectional fibers

P 2

¹ ^ 2 * 10 MN / m (foega Newtons / m): It was unexpectedly found that the graphitization increases the flexural strength by a factor of at least * 2, ie, up to one

2
Value of 500 iSJ / m.

The following Table I relates the carbon fiber content of the fiber-resin mixtures to the flexural strength, the interlaminar shear strength and the flexural modulus of the later carbonized material.

Tabel

Bend
strength
■ MN / m ² x Interlaminar
Shear strength
NM / m ² « Bending
module
GN / m ² ** Carbon fiber content
Weight X before
of carbonization 235 1.0 87 HO 270 8.5 101 50 2MO 11.0 123 60 160 3.0 152 70

* Mega Newton / m OH / » ² * Giga Newton / m ²

It was observed that preferred Paser levels for carbonized material in the range of 50 to 60 flew.-ί

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Table II below shows flexural strength values and flexural modulus for a range of fiber contents for carbonized material (R10993 resin) that is 26OO ° C had been subjected to graphitizing heat treatment.

Table II

Carbon fiber content flexural strength flexural modulus

_Λ ^G * "· - * Υ ⁰ ? MN / m ² « GN / rn ² **

of carbonization - ^u '

3Ö 245 132 40 317 140 50 400 150 60 500 180 70 575 220 x MN / » ² > Mega Newton / n ²

g ** OH / » ² s Olga Newton /» ²

It is evident that graphitization leads to a considerable increase in flexural strength; however, the flexural modulus shows only a moderate increase. Tue « Graphitization also showed a further slight one Weight loss, accompanied by a cross-section scraping and a decrease in the porosity of the matrix, resulting in a »

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. _u . 2122238

Increase in density. It has been observed that

added fiber content for graphitized material la BereiA from 60 to 70 wt. JC.

A further improvement in the flexural strength was observed when graphite is added to the uncured resin in the form of a powder dispersion: for example 20 wt. Jl graphite, added to phenolic resin, increases the flexural strength of a 60 wt. Ji fiber Press material from 500 MN / m to 828 MN / m ² .

Graphite can be added in amounts up to ¹ % by weight of the resin, 20% by weight is a preferred amount. As stated above, stiff additions of carbon black instead of graphite also reduce flexural strength and similar amounts can be used.

Instead of or in addition to a treatment of the cabbage - fibers by immersion in a solution of an oxidizing agent, as described in the preceding embodiments de Method according to the invention is described, the fibers can be immersed in a solution containing a carbide binding element. Table III below gives the interlaiiinar shear strengths of the carbonize Material again, the fibers of which initially

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BATH ORIGINAL

had been oxidized, and where the only variable is the solution treatment (carbonization temperature 100 ° C). The shear strength of a carbonized material formed from untreated fibers is also given for comparison. The numbers refer to molded parts with 60% by weight of Paser.

Tabel

III

Salt weight
on the fiber
<*) Shear strength
MMm ² - lbf / in ² (kp / cm ² ) 2630 (184.91) solution 25.0 18.1 2565 (180.34) NaCl 22.4 17.7 2390 (168.03) H ₃ BO ₃ 21.0 16.5 2365 (166.28) Ha ₂ SiO ₃ lt, 7 16.3 1360 (95.62) Be (NO ₃ ) ₂ - 9.4 -

* MN / m s Mega Newton / m

As indicated, fibers can be surface-oxidized by heating in air rather than by immersion in a solution of an oxidizing agent. A temperature in the range from 400 to 600 ° C is suitable for heating in air. "

Graphite articles according to the invention are, among other things, for an operation

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BATH ORIGINAL

at high temperatures, for example in Verbrennung3 ~ "hot zones ge ⁿ is, wherein an application possibility is given for ausschubTSystemen in missiles. The graphite material I3T to hot, corrosive gas and a heat cleavage against resistant.

Carbon fibers for use according to the invention can be either carbon fibers with a low modulus (carbonized) or carbon fibers with a high modulus (graphitized) . Such fibers can be obtained, for example, from polyacrylonitrile , as is known to the person skilled in the art. The flexural strength values as given above were obtained with a high modulus fiber.

In processing articles according to the invention , bundles of carbon fibers of long lengths are usually aligned side by side in a common direction, impregnated with resin and heated to partially cure the resin as described above. The partially cured fiber- resin mixture in the form of a thin film or a tape can be cut to suitable lengths and the cut lengths can be shaped. are wherein the fibers are aligned in a common direction, or, as it usually takes place, aligned according to a predetermined 3ind Lauster of "crossed layers", wherein

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the fibers of superimposed cut lengths have unequal directions. Articles which contain a pattern of "crossed plies" exhibit multifaceted strength properties according to the selected pattern.

In summary, the present invention relates to carbon fiber-reinforced graphite articles which are produced by impregnating carbon fiber bundles with a liquid containing a thermosetting resin, heating the impregnated bundles to convert the liquid to a viscous mass ("B" stage), compression molding of the partial hardened resin / fiber bundle for shaping, heating to convert the viscous resin to a hard-hardened resin, followed by a post-hardening heat treatment in air (at 200 ^° C.), further heating at 1000 ^° C. in an inert gas to carbonize the hardened one Resin fiber molded part, followed by vacuum and pressure impregnation with further liquid, hardening

and carbonizing the resin thus introduced - repeated if necessary - · to eliminate shrinkage and porosity in the carbonized material, and a final heat treatment at 260o ⁰ * C, in an inert gas for graphitization of the carbonized material. Preferred resins are phenolic resole resins or polyphenylene resins.

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Claims (5)

Patent a η s ρ rüche 1. A method for producing or processing a carbon fiber-reinforced graphite article, characterized in that it involves the impregnation of a bundle of aligned carbon fibers with a liquid containing a • thermosetting resin, the liquid having a time and Temperature increasing viscosity, heating the impregnated bundle to convert the contained therein Liquid into a viscous mass, by which the fibers are held together, shaping of the fiber bundle obtained, Heating the formed fiber bundle to hard-harden the resin and to solidify the viscous mass located between the fibers, heating the resin-hardened bundle in a non-oxidizing atmosphere for carbonization of the solidified mass and subsequent heating of the carbonized and shaped bundle in a non-oxidizing Atmosphere and at a higher temperature to the graphite sizing the carbonized material includes. 2. The method according to claim 1, characterized in that the carbon fibers of the bundle are surface-oxidized as the initial treatment. 3. Method according to claim 2, characterized in that 1098 * 7/18 * 0 BADOR ₁ G ₁ NAL _ ^ _ - Ib - the pasers can be made by heating them in air at an elevated temperature Surface oxidized. 4. The method according to claim 3, characterized in that the fibers in the air in a temperature range of Heated 400 to 600 ° C. ^ 5. The method according to claim 2, characterized in that the fibers are dipped in a solution of an oxidizing agent Surface oxidized. 6. The method according to claim 5, characterized in that the solution is an aqueous solution of nitric acid or Is sodium hypochlorite. 7. The method according to any one of claims 1 to 6, characterized characterized in that before the resin impregnation, the fibers w by immersion in a solution of a compound containing a C & rbid-forming element, surface-agility. 8. The method according to claim 7, characterized in that the carbide-forming element beryllium, boron, sodium or Silicon is. 9. The method according to any one of claims 1 to 8, characterized 109847/1840 bad OR _fGfNAL -17- characterized in that the fiber bundle by applying a Pressure on it is deformed. 10. The method according to claim 9 _t, characterized in that the forming pressure on the fiber bundle during the heating of the same for hard-hardening & the resin and solidification of the viscous mass is maintained. 11. The method according to any one of claims 1 to 10, characterized in that the thermosetting resin is a phenol-realeol resin, an epoxy resin, a furan, a xylenol-formaldehyde resin, a phenol novolak resin, a polyphenylene resin Polynaphthalene resin, a coal tar pitch, or a Mixture of two or more of these compounds is. 12. The method according to any one of claims 1 to 11, characterized in that after the carbonizing heat treatment impregnated the carbonized bundle with a further amount of the liquid in vacuo, as the impregnated The bundle is subjected to pressure, the introduced resin is heated to harden it and then in a non-oxidizing The atmosphere is heated at a higher temperature to carbonize the hardened resin. 13. The method according to any one of claims 1 to 12, characterized 109847/1840. . _l8 in that the bundle is further heated by the heating of the resin for hard-curing in the air at a higher temperature to improve the yield of carbon and aur promotion of dimensional stability during carbonization. Ik. Process according to Claim 13 »characterized in that the higher temperature is approximately 200 ^° C. 15 · Method according to one of Claims 1 to I ^, characterized characterized in that graphite or carbon black is added to the uncured resin and dispersed therein as a powder. 16. The method according to claim 15 »characterized in that graphite or carbon black in amounts of 0 bx3 UO wt. -J, based on the ₁ HaTz is added. 17. Carbon fiber-reinforced graphite article, produced 'by a method according to any one of claims 1 to 16. 18. A method for the production or further processing of a carbon fiber-reinforced graphite article according to claim and essentially as described above. 109847/1840 - 19 - - _19th 2122238 19. Carbon fiber-stiffened graphite article made by a method according to claim 1 and essentially as described above. 109847/1840