WO1996003279A1

WO1996003279A1 - Process for preparing novel carbonaceous fibers

Info

Publication number: WO1996003279A1
Application number: PCT/US1995/008913
Authority: WO
Inventors: Francis P. Mccullough, Jr.
Original assignee: The Dow Chemical Company
Priority date: 1994-07-21
Filing date: 1995-07-14
Publication date: 1996-02-08
Also published as: AU3009895A

Abstract

A process for preparing carbonaceous fibers for use as insulation. The process comprises the steps of subjecting polyacrylonitrile based fibers (12) to ionizing radiation (14) and then heat treating (16) the irradiated fibrous material to make the fibers (12) carbonaceous with an oxygen content less than 2 percent.

Description

PROCESS FOR PREPARING NOVEL CARBONACEOUS FIBERS

The present invention relates to a process for preparing novel carbonaceous fibers and to other materials prepared from the carbonaceous fibers. More particularly, there is provided a process which includes the step of subjecting to ionizing radiation thermoplastic polymeric fibers and preparing carbonaceous materials from the irradiated fibers without oxidative stabilization. The process of the invention provides fire resistant and thermal insulative materials.

Non-woven materials are made by the bonding of arrays of fibers or filaments. The materials can be made from staple fibers of discreet lengths by carding, wet laying, or the like.

Fibrous materials containing various carbonaceous fibers are, of course, well known in the prior art. Processes for preparing such fibrous carbonaceous materials from thermoplastic materials have been described in prior patents U.S. Pat. No. 5,168,004, which is incorporated herein by reference, discloses the preparation of multifilamentary carbonaceous material by the thermal stabilization and carbonization of meltspun acrylic multifilamentary material. The thermal stabilization results in cyclisization rather than crosslinking of acrylic material .

U.S. Pat. Nos. 4,837,076, 4,879,168 and 4,997,716 of McCullough et al, which are herein incorporated by reference, disclose crimped, irreversibly heat set, carbonaceous fibers which are derived from thermally stabilized polyacrylonitrile fibers.

Exposing polymeric materials to ionizing radiation to alter their properties is now known. The radiation embraces X-rays, gamma-and electron-radiation. These kinds of radiation are all essentially similar. Under exposure to radiation, free radicals or other reactive species are generated in the material. Ionizing radiation, for example from anelectron beam generator is known to create many complex and sometimes competing reactions. For example, radiation is known to induce polymerization of acrylonitrile to cause crosslinking. The radiation is most easily carried out at ambient temperature. This is no obstacle, however, to use of elevated temperatures, provided that the temperature is maintained below the temperature at which the material becomes tacky.

U.S. Patent No. 4,278,518 to Bjellqvist et al discloses a process for reducing monomer content of chlorinated hydrocarbon polymers by applying low dose ionizing radiation.

U.S. Patent No. 4,138,298 to Bobeth et al discloses the radiation of high polymer materials, including polyacrylonitrile to alter their physical characteristics. It is understood that the term "fibrous material" as used herein refers to conventional single or multiple strands of fibers formed by a spinning and drawing operation and fibers in the form or shape of a tow, fluff, web, batting, yarn or the like. The web or batting can also be in the form of a single ply or a multiplicity of superimposed or stacked plies.

The term "carbonaceous fibers" is understood to mean that the fibers have an oxygen content of less than 2 percent and that the carbon content of the irradiated fibers is greater than 65 percent by weight and the carbon content has been increased as a result of an irreversible chemical reaction generally caused by heat treating the fibers in a non-oxidizing atmosphere to render the fibers carbonaceous. Fibers having a carbon content of at least 92 percent are considered as being carbon fibers. Fibers having a carbon content of greater than 98 percent by weight are graphitic. The carbonaceous fibers are produced from polymeric precursors fibers such as, for example, polyacrylonitrile fibers which have been subjected to ionizing radiation. The fibers can be linear or non-linear.

The term "permanent" or "irreversibly heat set" used herein when applied to non-linear carbonaceous fibers relates to the fibers possessing a degree of resiliency and flexibility such that the carbonaceous fibers when stretched and placed under tension to a substantially linear shape but without exceeding the tensile strength of the fibers will revert substantially to their non-linear shape once the tension on the fibers is released. The foregoing terms also imply that the fibers can be stretched and released over many cycles without breaking the fibers.

The term "Pseudoextensibility" or "Pseudoelongatability" refers to the elongatability of a fiber which results from the crimped or non-linear configuration including any false twist that is imposed on the fiber.

The term "bending strain of the crimped fiber" as used herein is as defined in Physical Properties of Textile Fibers. .E. Morton and J. .S. Hearle, The Textile Institute, Manchester, 1975, pages 407-409. The percent bending strain resulting from the crimp on the fiber can be determined by the equation:

S = r/R x 100 where S is the percent (%) bending strain, r is the fiber radius and R is the radius of curvature of bend. That is, if the neutral plane remains in the center of the fiber, the maximum percentage tensile strain, which will be poositive on the outside and negative on the inside of the bend of the fiber, equals r/R x 100 in a circular cross section of the fiber.

According to the present invention there is provided a process for preparing novel carbonaceous fibers. The process comprises the steps of stabilizing by exposure to ionizing radiation a precursor fibers comprising spun and drawn polyacrylonitrile fibers, and then heat treating the irradiated fibers in an inert atmosphere to render them carbonaceous, particularly, non- oxocarbonaceous . That is, the carbonaceous fibers have less than 2 percent oxygen content .

The precursor fibers comprising polyacrylonitrile fibers can be prepared by conventional spinning and drawing processes . The fibrous material containing polyacrylonitrile fibers is subjected to ionizing radiation, preferably electron radiation of at less than 10 megarads, preferably less than 2 megarads, more preferably 0.1 to 1.5 megarads, so as to crosslink the polyacrylonitrile fibers or filaments and stabilize them without an oxidizing step for the subsequent heat treatment step which results in their carbonization. Little or no oxygen is pushed up by the fibers during their preparation. The resultant carbonaceous fibers or filaments of the fibrous material have a tenacity of from 2 to 20 grams/denier (g/d) , preferably from 6 to 19 g/d.

The fibrous carbonaceous material of the invention in the form of a batting generally has a bulk density of from 100 to 300 cc/g (cubic cm per gram) or, conversely, 0.01 to 0.003 g/cc (grams per cubic cm) , preferably from 200 to 300 cc/g (.005 to .003 g/cc) .

The preferred non-linear carbonaceous fibers of the invention are characterized by having a multiplicity of crimps along their length and by having an elongatability to break of from 2 to 9 percent, a pseudoelongatability of from 0.2 to 18 percent, and a bending strain value of less than 50 percent, preferably less than 30 percent. The carbonaceous fibers of the invention, such as in the form of a batting, are particularly useful to provide high thermal insulation, typically greater than 3 R/in, preferably 5-6 R/in, where R is measured in hr»ft²» -F/BTU. The fibrous carbonaceous material prepared with the fibers of the invention is also particularly useful as a fire resistant and ignition resistant insulation and can be used in lieu of fiberglass or other forms of insulation for buildings. The fibrous carbonaceous material of the invention can also be used as thermal and ignition resistant insulation or padding for articles for personal use such as gloves, jackets, sleeping bags, etc., as furniture upholstery and covers, curtains, comforters, mattress pads, etc., as padding for carpeting, and the like.

It is therefore an object of the invention to provide a novel process for preparing an ignition resistant, thermal insulating material utilizing as a precursor non-oxidized polyacrylonitrile fibers.

It is also an object of the invention to provide a process for preparing a fire resistant material of carbonaceous fibers which can be used alone or in combination with other fibers.

It is another object to provide ignition resistant material which can be used as thermal insulation in buildings, and the like. The objects and advantages of the invention will become more clearly understood from the drawing and the description of the preferred embodiments. Brief Description of the Drawings

Figure 1 is a schematic view of a continuous process for preparing the carbonaceous fibers of the invention. Description of the Preferred Embodiments

As illustrated in Figure 1, polyacrylonitrile based fibers 12 are extruded from a conventional spinning and drawing apparatus 10 onto a conveyor 19. The fibers 12 are deposited on the conveyor 19 and passed through a source 14 of ionizing radiation, such as electron beam radiation so as to stabilize the fibers by crosslinking. The irradiated fibers are heat treated in an oven 16 in an inert atmosphere and preferably without tension as disclosed in U.S. Pat. No. 4,837,076 at a temperature to render them carbonaceous. The fibers 12 are then collected on a collector or take-up roll 18 for further processing.

While the process shown in Figure 1 can be used beginning from the initial extrusion, it can be readily adapted to be performed batchwise. That is, the process can begin utilizing a creel of spun fibers, for example, a tow of polyacrylonitrile fibers, which has been crosslinked separately by ionizing radiation at less than 10 megarads, preferably between 0.1 to 1.5 megarads. The crosslinked fibers are passed through the heat treatment oven for carbonization.

The irradiated fibers can be linear, non-linear, single stranded or in the form of a tow, yarn, fluff, batting, or the like.

If desired, a tow of fibers 12 during heat treatment can be subjected to a dynamic flow of inert non- oxidizing gas passing through the tow 12. The dynamic flow of gas passing through the tow during heat treatment improves the tenacity of the fibers by removing intersticial oxygen and other gases. Preferably, the gas is nitrogen.

A plurality of extrusion dies can be positioned in a sequential manner downstream of a first nozzle to provide a plurality of juxtaposed layers or plies of fibrous material that can be positioned one on top of the other to provide a structure, that is a batting, of any desired loft. However, if a batchwise process is employed, the batting can be first prepared and then sent through a radiation zone separately.

The batting of the invention, preferably in the form of one or more plies, can be supplied in any desired thickness depending on the particular use to be made of the batting and can have a thickness from 4 to 100 millimeters. The density of the batting can also vary widely depending on the particular uses to which the batting is applied. Generally, the batting has a density of at least 100 cubic centimeters/gram (cc/g) .

Natural or synthetic non-carbonaceous fibers can be blended with from 7 to 20 percent by weight of the fibrous carbonaceous material of the invention to produce a non-flammable insulation. An increase in the amount of the carbonaceous material above 20 percent by weight, based on the total weight of the insulation, further improves the fire resistance of the insulation. Insulation materials which contain the carbonaceous fibers in an amount of from 50 to 90 percent by weight have fire blocking characteristics. Fibrous materials, such as battings, which contain high amounts, for example, 90 percent by weight of the carbonaceous fibers, are particularly suitable for use as ceiling insulation in buildings in extreme climates and/or in structures to be insulated against radiant energy. When carbonaceous fibers are used that have a relatively high electrical conductivity, an insulation containing such fibers provides electromagnetic radiation shielding, such as in shielding from microwaves

In accordance with a further embodiment of the invention, the battings which are prepared with the fibers of the invention may be thermally bonded with a thermoplastic binder and then subjected to heat and pressure as disclosed in U.S. Pat. No. 4,997,716 to form a structural panel or board.

In accordance with a still further embodiment of the present invention, a ceramic and/or metallic coating can be formed on the fibrous carbonaceous material in the form of a fiber or filament per se or a fiber assembly, i.e., a plurality of fibers or filaments such as in the form of a mat, batting, yarn or fabric. The coated fibrous carbonaceous material may advantageously be used in oxidation conditions and at high temperature applications wherein uncoated carbonaceous fiber substrates could otherwise not be used satisfactorily.

The ceramic materials which can be utilized in the present invention comprises the oxides or mixtures of oxides, of one or more of the following elements: magnesium, calcium, strontium, barium, aluminum, scandium, yttrium, the lanthanides, the actinides, gallium, indium, thallium, silicon, titanium, zirconium, hafnium, thorium, germanium, tin, lead, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, and uranium. Compounds such as the carbides, borides and silicates of the transition metals may also be used. Other suitable ceramic materials which may be used are zircon-mullite, mullite, alpha alumina, sillimanite, magnesium silicates, zircon, petalite, spodumene, cordierite and alumino- silicates. Suitable proprietary products are "MATTECEL" (Trade Name) supplied by Matthey Bishop, Inc., "TORVEX" (Registered Trademark) sold by E.I. du Pont de Nemours & Co., "Wl" (Trade Name) sold by Corning Glass and "THERMACOMB" (Register Trademark) sold by the American Lava Corporation. Another useful product is described in British Patent No. 882,484.

Other suitable active refractory metal oxides include forexample, active or calcined beryllia, baria, alumina, titania, hafnia, thoria, zirconia, magnesia or silica, and combination of metal oxides such as boria- alumina or silica-alumina. Preferably the active refractory oxide or metal is composed predominantly of metals or oxides of one or more metals of Groups II, III and IV of the Periodic Table. Among the preferred compounds may be mentioned

YC, TiB₂, HfB₂, VB₂, VC, VN, NbB₂, NbN, TaB₂, CrB₂, MoB₂ , and W₂B.

Preferably, the coating formed on the surface of the fibrous carbonaceous material of the present invention are selected from oxides such as Ti0 ; nitrides such as BN; carbides such as BC and TiC; borides such as TiB₂ and TiB; metals for example Ni, Au, and Ti; and the like.

Any conventional method of forming the coating on the fibrous carbonaceous material of the invention may be used. For example, a chemical vapor deposition can be used. The fibrous carbonaceous material can be dipped into a coating solution to form the coating. Brushing or spraying a coating solution on the fibrous carbonaceous material can also be used.

The thickness and amount of coating applied to the fibrous carbonaceous material should be sufficient such that the surface coating substantially insulates the fibrous substrate from the oxygen-containing atmosphere, i.e., such that the coating exposed to the oxygen- containing atmosphere protects the fibrous material from oxidation. The thickness and amount of coating on the fibrous carbonaceous material will depend on the form in which the fibrous material is used and the desired application for the fibrous material. For example, the coating thickness will depend on whether the fibrous carbonaceous material is a single ply which can have a coating thickness of 1 micron or a batting which can have a coating thickness of 10-100 microns.

In those structures of the invention that are used as fire barriers, conformal silicone compounds, such as are commercially available from the Dow Corning

Corporation, can be used as coatings on the carbonaceous fibers to synergistically improve their fire barrier performance as described in U.S. Pat. No. 4,950,540. An amount of the silicone compounds used is generally 0.5 to 20 percent by weight.

Preferably, the stabilized polymeric precursor fibers used to prepare the carbonaceous fibers are derived from acrylic filaments, preferably polyacrylonitrile (PAN) filaments. The acrylic filaments are selected from one or more of the following: acrylonitrile based homopolymers, copolymers and terpolymers. The copolymers preferably contain at least 85 mole percent of acrylonitrile units and up to 15 mole percent of one or more monovinyl units. Examples of vinyl monomers copolymerizable with acrylonitrile include methacrylic acid esters and acrylic acid esters such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate and ethyl acrylate; vinyl esters such as vinyl acetate and vinyl propionate,* acrylic acid, methacrylic acid, maleic acid, itaconic acid, and the salts thereof; vinylsulfonic acid and the salts thereof . When the non-graphitic carbonaceous fibers of the invention are derived from polyacrylonitrile based materials, they can be classified according to carbon content and electrical conductivity analogous to the three groups disclosed in U.S. Pat. Nos. 4,950,533 and

4,950,545. The non-graphitic carbonaceous fibers have a nitrogen content between 5 and 35 percent.

In a first group, the carbonaceous fibers are partially carbonized and have a carbon content of greater than 65 percent but less than 85 percent, are electrically non-conductive and do not possess any electrostatic dissipating characteristics, i.e., they are not able to dissipate an electrostatic charge.

The term electrically nonconductive as utilized in the present invention relates to fibers having a specific resistivity of greater than 10^"1 ohm-cm. The specific resistivity of the fibers is calculated from measurements as described in U.S. Pat. No. 4,898,783, issued February 6, 1990 to McCullough et al. When the fiber is a crosslinked and heat set acrylic fiber it has been found that a nitrogen content of 22 percent or higher results in an electrically nonconductive fiber.

In a second group, the carbonaceous webs are classified as having low electrical conductivity, that is being partially electrically conductive and having a carbon content of greater than 65 percent but less than 85 percent. Low conductivity means that the fibers have an electrical resistivity between 10"-*^• and 10" ohm-cm. When the carbonaceous fibers are derived from irradiated acrylic fibers and have a low conductivity, they possess a percentage nitrogen content of from 16 to 22 percent, preferably from 16 to 18.8 percent.

In a third group, the fibers have a carbon content of at least 85 percent and a nitrogen content of 5 to 15 percent. These fibers are characterized as having a high electrical conductivity. That is, the fibers have an electrical resistivity of less than 10"⁴ ohm-cm.

Having thus broadly described the present invention and a preferred embodiment thereof, it is believed that the same will become even more apparent by reference to the following examples. It will be appreciated, however, that the examples are presented solely for purposes of illustration and should not be construed as limiting the invention. Example 1

A 320K tow of spun and drawn polyacrylonitrile fibers copolymerized with 5 percent by weight of maleic acid was cut into7.5 cm length staple. The staple was then carded on a PlattMiniature carding machine to produce a wool like fluff having fibers ranging from 6.5 to 7.5 cm in length.

This fluff was then irradiated at E-Beam Corporation, Cranbury, N.J. with ionizing radiation at 1.5 megarads to crosslink the polymer. The crosslinked fluff was then heat treated in a non-oxidizing atmosphere at

400-900*C under a dynamic flow of nitrogen to permanently heat set the polymer and render the polymer carbonaceous. The resulting carbonaceous fluff was tested for ignition resistance by FTM 5903 and FAR 25.853-b vertical burn tests. There was no afterburn, and a char length of less than 1 inch was formed with no droppings. The electrical resistivity of the fibers in the batting was 10^~-*- ohm- cm. The batting was useful as building insulation. Example 2

A 6K tow of linear polyacrylonitrile fibers wound on a creel was transported to a source of electron beam radiation at E-Beam Corporation where they were subjected to radiation at 1.5 megarads to crosslink the fiber. The fibers were then heat treated at 650*C in an inert atmosphere under the conditions described in Pat. No. 4,879,168 to be rendered carbonaceous. The resultant fibers had a nitrogen content of 22 percent and a specific resistivity greater than 10^-1 ohm-cm. Example 3

The heat set linear tow of Example 2 was cut into 7.5 cm staple length, opened with turbulent air and blended with 75 percent of 6 denier polyester fiber and made into a 2.5 cm thick batting having a density of approximately 9 kg/m³. This batting was then thermally bonded in a Benz hot air oven. This material passed the vertical burn test as described in 14 CFR 25.853b (herein incorporated by reference) and had a specific resistivity of less than 10"¹ ohm-cm (in the effective anti-stat range) . This material was shown to be effective as a flame retardant material useful as an anti-stat flame arresting structure.

Claims

What is Claimed:

1. A method for preparing carbonaceous fibers which comprises the steps of: 1) exposing to ionizing radiation spun and drawn polyacrylonitrile based fibers so as to cause crosslinking of said polyacrylonitrile fibers, and then

2) heat treating said irradiated fibers in an inert atmosphere so as to increase the carbon content of said irradiated fibers to form said carbonaceous fibers.

2. The method of claim 1 wherein said polyacrylonitrile based fibers are selected from the group consisting of linear fibers, non-linear fibers and a mixture thereof.

3. The method of claim 1 wherein said radiation is less than 10 megarads.

4. The method of claim 3 wherein said radiation is 0.1 to 1.5 megarads.

5. The method of claim 1 including the step of subjecting said irradiated fibers to a dynamic flow of non-oxidizing gas during heat treatment.

6. The method of claim 1 wherein said irradiated fibers are heat treated at a temperature and for a period of time so as to form graphitic material.

7. The method of claim 1 wherein said irradiated fibers are heat treated at a temperature and for a period of time so as to be non-graphitic.

8. The method of claim 1 wherein said ionizing radiation is electron radiation.

9. Carbonaceous fibers prepared according to the process of claim 1.

10. The fibers of claim 9 in the form of a batting.

11. The fibers of claim 9 in the form of a board.

12. The fibers of claim 9 which is coated with ceramic material.

13. The fibers of claim 9 which is coated with metal.

14. The fibers of claim 9 which is coated with a conformal silicone compound.

15. The fibers of claim 9 comprising non-graphitic carbonaceous fibers having a nitrogen content of 5 to 35 percent and an oxygen content less than 2 percent.

16. A fibrous carbonaceous material from fibers prepared by the process of claim 3.

17. A fibrous carbonaceous material from fibers prepared by the process of claim 6.

18. A fibrous carbonaceous material from fibers prepared by the process of claim 7.

19. A fibrous carbonaceous material from fibers prepared by the process of claim 8.