DE68912629T2 - Dense structures made of carbon fibers. - Google Patents

Abstract

A fibrous structure comprising a multiplicity of nonflammable, nonlinear, substantially irreversibly heat-set, first carbonaceous polymeric fibers, and at least one second nonflammable carbonaceous polymeric fiber, yarn or tow in an interlocking relationship with said first fibers; the fibrous structure is preferably densified and has a bulk density of from 4.8 to 32 kg/m<3>, and a process for producing the interlocked, fibrous structure useful as a thermal insulating and/or sound absorbing structure.

Description

The present invention relates to a method for producing a compacted fiber structure from a plurality of firstly non-combustible, non-linear, elastic, extendable, essentially irreversibly heat-set carbon polymer fibers by joining the fibers to second fibers of a carbon polymer precursor material and subsequently heat-treating the entire structure to heat-set the second fibers.

The fiber structure according to the invention is used in thermal or noise insulation and in filtration. The structures are densified and characterized by good shape and volume retention and are structurally stable against numerous compression and decompression cycles. These structures with a relatively high densification (compared to non-densified structures) surprisingly have a felt-like appearance and few broken fibers.

A variety of carbon fibers are used to make wool-like fluff, felt, fabric, blankets, batting and the like. These are hereinafter referred to generally as a "fiber structure" for brevity. Where the fiber structure is densified, such as by implanting a second fiber, the structure is generally referred to as a "densified fiber structure" or simply a "densified structure."

The term "implanting" as used herein generally refers to a process for interlocking, mixing or bonding fibers together. The fiber structure of the first Fiber is preferably densified by stitching the fiber structure to the second fiber.

For many high temperature insulation applications it is desirable to produce a fiber structure, such as a wool-like fluff or batting, of higher density so that it retains its integrity and densified structure during prolonged exposure to higher temperatures. At temperatures above 400ºC densified structures are useful and retain their good mechanical and physical properties.

Non-combustible, non-linear, elastic carbon fibers suitable for making the fibrous structures of the present invention are described in EP-A-0 199 567, published on October 29, 1986, entitled "Carbonaceous Fibers and Springlike Reversible Deflection and Method of Manufacture" by McCoulough et al. Prior to the present invention, it was not possible to permanently densify a fibrous structure of the above-mentioned non-linear carbon fibers and to maintain the integrity of the densified fibrous structure at temperatures above 400°C.

At temperatures above 400ºC, fibers made of the above-mentioned carbon polymer material (including non-combustible p-aramid fibers) decompose and the fiber structure consequently loses its integrity. It is therefore of considerable advantage to be able to permanently densify and bond a fiber structure to a polymer fiber that does not lose its physical properties at elevated temperatures.

US-P-4,628,846 discloses an apparatus that can be used to produce the densified structures of the invention.

The present invention relates to a densified fiber structure comprising a plurality of non-combustible, non-linear, substantially irreversibly heat-set first carbon polymer fibers, wherein the first fibers are elastic, shape-recoverable and elongatable and have a reversible bend ratio of greater than 1.2:1 and an aspect ratio of greater than 10:1, and at least one second non-combustible, substantially irreversibly heat-set carbon polymer fiber, yarn or tow implanted in an interconnecting manner with the first fiber to form an interconnected fiber structure.

The present invention preferably relates to a fiber structure in which the first carbon fibers have a sinusoidal or coiled configuration and the fiber structure is in the form of at least one layer of a non-woven, wool-like fluff, batting or fabric and the second carbon fibers, yarns or tows have a linear or non-linear configuration and have a higher denier than the first carbon fibers.

The second connecting carbon fibers are advantageously chemically similar or identical to the first carbon fibers of the fiber structure.

The invention further relates to a compacted fiber structure with a bulk density of 4.8 to 32 kg/m³.

The present invention further relates to a method for producing a fibrous structure from a plurality of non-combustible, non-linear, substantially irreversibly heat-set, first carbon polymer fibers, which comprises the steps of implanting into the first fibers at least one non-heat-set second carbon fiber, yarn or tow in a connecting relationship with the first fiber and subsequently treating the fibrous structure with heat in an inert atmosphere in order to heat-set the connecting second fiber, yarn or tow.

The method according to the invention also enables the fiber structure of the first fibers to be mixed with second carbon polymer precursor fibers of larger diameter, which have a greater shear resistance during implantation, for example a needle punching process. Carbon fibers with a relatively larger denier number can also be provided for greater mechanical strength.

According to a preferred embodiment of the invention, a fiber structure of the first heat-set non-linear carbon fibers is implanted by needle punching with the second fiber, yarn or tow made from a carbon precursor material to increase the bulk density and mechanical strength of the fiber structure. The needle punching causes the second fibers to form loops in the fiber structure.

Heat treatment of the fiber structure then ties the loop stitch. A high degree of needle punching can be used to form a densified structure that has a felt-like feel and appearance after heat treatment.

According to another embodiment of the invention, two or more fiber structures, such as batting, can be connected to one another. The fibers of one batting can be used as connecting fibers for the other fibers.

The first carbon fibers preferably have a sinusoidal or coiled configuration or a more complex structural combination of the two. These first fibers may also comprise linear, heat-set carbon polymer fibers.

The carbon fibers used in accordance with the invention have a carbon content of at least 65%, a nitrogen content of 5 to 35% and a LOI value of greater than 40. In determining the specific use for which To determine the application for which they are most suitable, these fibres are identified in particular by their degree of carbonification and/or their ability to conduct electricity.

The first carbon fibers or matrix fibers are prepared by heat treating a suitable stabilized carbon precursor material, such as stabilized polyacrylonitrile (PAN) or pitch-based materials, for example petroleum or coal tar pitch-derived materials, or other polymeric materials that can be converted into non-combustible or thermally stable carbon fibers or fiber structures.

For example, in PAN-based fibers, the fibers are prepared by melting or wet spinning a suitable liquid of a precursor material having a nominal diameter of 4 to 25 µm. The fibers are collected as a collection of a plurality of continuous filaments into yarns and stabilized by oxidation, in the case of PAN-based fibers in a conventional manner. The stabilized fibers, yarns or staple yarns (made from chopped or stretch-broken fiber staples) are then formed into a ball-like and/or sinusoidal shape by combining the fibers, yarn or tow into a fabric or cloth (it is clear that other cloth-forming and ball-forming processes may be used).

The fabric or material thus formed is then heat treated in a relaxed, stress-free state at a temperature of 525º to 750ºC in an inert atmosphere for a period of time to effect a heat-induced thermosetting reaction in which an additional cross-linking and/or an inter-chain cyclization reaction takes place between the original polymer chain. At the lower temperature range of 150º to 525ºC, the chains are provided with a varying degree of temporary to permanent fixation, while the fibers in the upper range from 525º to 750ºC to a substantially permanent or irreversible heat set configuration. The term "permanently" or "irreversibly heat set" means that the carbon fibers possess a degree of irreversibility such that the non-linear fibers, when stretched to a substantially linear configuration without exceeding their internal tensile strength, return to their non-linear configuration when the tension on the fiber is removed. Fibers heat treated according to the above process can be stretched to a substantially linear configuration and return to their stress-free, non-linear configuration when the stress is removed. Such stretching of the fiber can be carried out over many cycles without breaking the fiber, even when additional stress (without exceeding the tensile strength of the fiber) is applied to the fiber when it is already in a substantially linear configuration.

It will be understood, however, that the fibers may be initially heat treated at a higher temperature range, provided that the heat treatment is carried out under an inert, non-oxidizing atmosphere and while the skein-like or sinusoidal fibers are in a relaxed or stress-free state. As a result of treatment at higher temperatures of 525º to 750ºC, the fibers, yarn or tow are provided with a permanently fixed sinusoidal or skein-like configuration. The fibers, yarn or tow thus obtained, having the non-linear structural configurations obtainable by unravelling a woven fabric, are subjected to other processes known in the art to effect opening, a process in which the yarn or fibers of the fabric are separated into an entangled, wool-like, fluffy material in which the individual fibers retain their ball-like or sinusoidal configuration, resulting in a fiber structure with a considerable amount of open space.

The stabilized fibers, permanently fixed in their desired structural configuration by, for example, bonding and then heating in a relaxed and stress-free state to a temperature above 525ºC, retain their elastic and reversible bending properties. It will be understood that higher temperatures of up to about 1500ºC may be used, but the most flexible fibers and the least loss due to fiber breakage when the fibers are coated to produce the fluff are found in fibers heat treated at a temperature of 525º to 750ºC.

The second carbon fibers used in the present invention include fibers that combine with the first fibers of the fiber structure described above and that can withstand the high temperatures described. The second fibers may be from a different thread, may be fibers of a similar batting, or may be mixed into the first fibers to form the wool-like fluff or batting and used for densification.

The connecting second fibers may preferably be made from a similar stabilized carbon precursor polymer material as the first fibers. A suitable stabilized precursor material may be selected, for example, from PAN or pitch-based materials (e.g., petroleum or coal tar) or other polymeric materials that are thermally stable at the high temperatures of interest described above, such as aramid fibers, particularly the aromatic polyaramids, for example KEVLAR (a trademark of E.I. du Pont de Nemours & Co., Inc.).

PAN-based fibers can be collected as a collection of a plurality of continuous filaments in yarns and stabilized by oxidation in a conventional manner. The stabilized second fibers, yarns or staple yarns (made from cut or stretch-broken staple fibers) are then implanted according to the invention into the first carbon fiber structure and form the fiber structure or a compacted structure.

When implanted into the fiber structure, the second carbon fibers can be incorporated into the structure as linear or non-linear fibers before heat-setting the second fiber.

The second, non-linear fibers can be manufactured in a similar manner to the first fibers by temporarily fixing the fibers in a relaxed and stress-free state at a temperature range of 150º to 525ºC by heat treatment in an inert atmosphere. The fibers are provided with a varying degree of temporary to permanent fixation as the temperature increases in the specified temperature range. The fibers are then permanently fixed by chemical treatment or by heat treatment of the fiber structure after the bonding step. The heat treatment is preferably carried out at a temperature of 525ºC and higher so that the fibers are provided with a permanent fixation.

When the second carbon fibers are permanently heat-set, the fiber structure has an integrity and handleability that encompasses the combination of the first and second carbon fibers.

As with the first fibers, temperatures of up to 1500ºC can be used to heat set the second fibers, but the most flexible and the least loss due to fiber breakage are found in the fibers heat set at a temperature of 525ºC to 750ºC.

The bonded fiber structure is used in thermal insulation at high temperatures and in noise absorbing structures and can be used depending on the specific use and the environment in which the structures are placed into 3 groups.

In a first group, the carbon fibers used in the fiber structure of the invention are electrically non-conductive. The term non-conductive refers to a resistance of greater than 4x10⁶ ohm/cm when measured on a 6K tow of fibers each having a diameter of 7 to 20 µm. The resistivity of each fiber is greater than about 10⁶ ohm/cm.

In a second group, the carbon fibers used in the fiber structure according to the invention are classified as partially conductive (i.e. they have a low electrical conductivity) and have a carbon content of less than 85%. If the stabilized precursor fiber is an acrylic fiber, i.e. a PAN-based fiber, the nitrogen content is 5 to 35%, preferably 16 to 20%. These partially conductive fibers are excellent for insulating aircraft and for insulating areas where public safety is a concern. The structures formed therefrom are light, absorb moisture only to a small extent, have good abrasion resistance and have a good appearance and good handleability.

The higher the carbon content in the carbon fiber, the greater the electrical conductivity. Such fibers retain a wool-like appearance when incorporated into a compacted structure, particularly when the majority of the fibers are non-linear, i.e., skein-like fibers. In addition, the higher the percentage of skein-like fibers in the structure, the greater the elasticity of the structure. As a result of the higher carbon content, structures made from these partially conductive fibers also have greater noise absorbing properties and more effective thermal insulation properties at higher temperatures. These fibers have an electrical resistance from 4x10⁶ to 4x10³ Ohm/cm when measured on a 6K tow of fibers with a respective diameter of 7 to 20 μm.

In a third group are carbon fibers with a carbon content of at least 85%. Due to their high carbon content, these fibers have superior thermal insulation and noise absorbing properties. The coil-like or sinusoidal shape of the fibers in the fiber structure provides insulation that provides good compressibility and elasticity while maintaining improved thermal insulation. The fiber structure made with fibers of the third group is particularly useful in insulating furnaces and in areas of high heat and high noise levels.

The fibers of the third group that are used are preferably derived from stabilized acrylic fibers and have a nitrogen content of less than 10%. As a result of the still higher carbon content, the fiber structures are more electrically conductive. This means that the electrical resistance is less than 4x10³ Ohm/cm when measured on a 6K tow of fibers with a respective diameter of 7 to 20 um.

The stabilized precursor acrylic fibers advantageously used to make the fiber structures are selected from acrylonitrile homopolymers, acrylonitrile copolymers, and acrylonitrile terpolymers. The copolymers preferably contain at least about 85 mole percent acrylonitrile units and up to 15 mole percent of one or more monovinyl units copolymerized with styrene, methyl acrylate, methyl methacrylate, vinyl chloride, vinylidene chloride, vinylpyridine, and the like. The acrylic filaments may also contain terpolymers in which the acrylonitrile units preferably make up at least about 85 mole percent.

It is further understood that the carbon precursor starting materials can impart electrical conductivity in the range of metallic conductors by heating a fibrous structure to a temperature above about 1000°C in a non-oxidizing atmosphere. The electrically conductive property can be obtained from selected starting materials such as pitch (petroleum or coal tar), polyacetylene, acrylonitrile-based materials, for example a polyacrylonitrile copolymer (PANOX or GRAFIL-01), polyphenylene, polyvinylidene chloride (SARAN, trademark of Dow Chemicals) and the like.

According to the invention, antistatic fibers, i.e. fibers that have the ability to relieve electrostatic charge and that also serve as connecting and compacting fibers, can be incorporated into the fiber structure.

Preferred precursor materials are prepared by melt spinning or wet spinning the precursor materials in a known manner to yield a monofilament fiber or multifilament tow. The fibers, yarn or tow are then incorporated into a woven fabric or bonded web using any of the known commercially available techniques. The fabric or web is then heated to a temperature above 525°C, preferably above 550°C, then cut and carded to produce the wool-like fluff used in the fiber structure of the present invention.

If desired, the densified fiber structure can be treated with heat to form carbon or graphite structures. The present process allows the production of carbon or graphite structures without complicated joining processes.

It is clear that all percentages used herein refer to percentages by weight.

Particular embodiments of the present invention are illustrated in the following examples.

example 1

A. A non-linear fiber tow heated to 550ºC was opened with a Shirley opener and blended with 25% large denier dogbone OPF (oxidized PAN fiber) obtained from RK Carbon Fibers Inc., Philadelphia, Pennsylvania. The dogbone OPF, which was heated to 200ºC prior to blending, had a temporary crimp. Batts were assembled, run through a needle punch machine and densified from 7.5 cm to 1.8 cm thickness with the same precursor fibers.

B. The resulting densified batting or felt from Part A, containing the dogbone-shaped OPF stitches, was heat treated under nitrogen at 700ºC for 60 minutes. The resulting densified batting or felt had good permanent integrity and was stable at temperatures up to over 400ºC.

Example 2

A compacted cotton was prepared according to the procedure of Example 1A. The cotton thus obtained was then heat-treated at a temperature of 1500°C for 60 minutes to form a uniform carbon structure suitable for noise and thermal insulation.

Claims (21)

1. A densified fibrous structure comprising a plurality of non-combustible, non-linear, substantially irreversibly heat-set, first carbon polymer fibers, wherein the first fibers are elastic, shape-recoverable and extensible and have a reversible bend ratio of greater than 1.2:1 and an aspect ratio of greater than 10:1, characterized in that the structure further comprises at least one second non-combustible, substantially irreversibly heat-set, carbon polymer fiber, yarn or tow implanted in an interconnecting relationship with the first fibers to form a densified, interconnected fibrous structure. 2. Structure according to claim 1, wherein the interconnecting fiber structure has a bulk density of from 4.8 to 32 kg/m³. 3. The structure of claim 1, wherein the first and second carbon fibers are derived from stabilized polymeric precursor fibers or from pitch-based precursor fibers having a diameter of from 4 to 25 µm. 4. The structure of claim 3 wherein the polymeric precursor fibers are acrylic fibers selected from acrylonitrile homopolymers, acrylonitrile copolymers, and acrylonitrile terpolymers, wherein the copolymers and terpolymers contain at least 85 mole percent acrylic units and up to 15 mole percent of one or more monovinyl units copolymerized with another polymer. 5. A structure according to any preceding claim, wherein the carbon fibers have a carbon content of more than 65% and an LOI value of more than 40. 6. The structure of claim 5, wherein the carbon fibers are electrically conductive and have a carbon content of at least 85% and an electrical resistivity of less than 4x10³ ohm/cm, measured on a 6K tow of fibers each having a nominal diameter of 7 to 20 µm. 7. The structure of claim 5, wherein the carbon fibers are not electrically conductive and do not have electrostatic dissipating properties, have a carbon content of less than 85 percent and an electrical resistivity of greater than 4x10³ ohm/cm, measured on a 6K tow of fibers each having a nominal diameter of from 7 to 20 µm. 8. The structure of claim 5, wherein the carbon fibers have low electrical conductivity and electrostatic dissipating properties, a carbon content of less than 85 percent, and an electrical resistivity of 4x10⁶ to 4x10³ ohm/cm, measured on a 6K tow of fibers each having a nominal diameter of 7 to 20 µm. 9. A structure according to any preceding claim, wherein the first carbon fibers have a sinusoidal or a ball-like configuration and the fiber structure is in the form of a non-woven, wool-like fluff, batting or cloth and the second carbon fiber has a linear or non-linear configuration and a higher denier than the first carbon fiber. 10. A method for producing a fiber structure from a plurality of non-combustible, non-linear, essentially irreversibly heat-set, first carbon polymer fibers, characterized by implanting at least one non- heat-set second carbon polymer fiber, yarn, or tow into the first fibers in an interconnecting relationship with the first fibers, and subsequently heat treating the fiber structure in an inert atmosphere to heat-set the interconnecting second fiber, yarn, or tow. 11. The method of claim 10, wherein the second fiber, yarn or tow is made from a precursor carbon polymer material that can be irreversibly heat-set to form a carbon fiber, yarn or tow that is similar or identical in composition to the first heat-set carbon fibers. 12. A method according to claim 10 or 11, wherein the fibers are acrylic fibers, and including the step of heat treating the fibrous structure containing the second non-heat-set interconnecting fibers in an inert atmosphere at a temperature above 525°C to provide the second fiber, yarn or tow with a permanent set. 13. A method according to claims 10, 11 or 12, wherein the fibrous structure is in the form of a wool-like fluff, mat, felt or batting, and the second fiber, yarn or tow is present in the fibrous structure to densify the structure to a bulk density of from 4.8 to 32 Kg/m³ to provide the structure with integrity and handleability. 14. A method according to any one of claims 10 to 13, wherein the implantation of the second fibers into the first fibers is achieved by needle punching. 15. A method according to any one of claims 10 to 14, wherein the second carbon fibers are selected from fibers having the same or a different composition than the first carbon fibers. 16. A method according to any one of claims 10 to 15, wherein the implanted second fiber is derived from a cotton wool. 17. A method according to any one of claims 10 to 16, wherein the second fiber is a linear or non-linear fiber. 18. A method according to any one of claims 10 to 17, wherein the fibrous structure comprises a plurality of batts. 19. A method according to claim 18, wherein at least one batt comprises carbon fibers having a carbon content of at least 85%. 20. The method of claim 18, wherein at least one batt includes linear fibers. 21. A method of making a diverse batt structure comprising the steps of providing a first batt of a first non-linear, elastic, shape-recoverable, extensible, non-combustible, heat-set carbon fiber derived from oxidized polyacrylonitrile, the fibers having a reversible bend ratio of greater than 1.2:1, an aspect ratio of greater than 10:1, and a limited oxygen index value of greater than 40, overlaying at least a second batt of polyacrylonitrile fibers on the first batt, bonding the polyacrylonitrile fibers of the second batt to the heat-set fibers of the first batt, and then treating the entire densified structure with heat to substantially permanently heat-set the second batt.