Templated carbon fibers and their application

Abstract

Disclosed is a carbon fiber with a template derived shape and method for making the same by (a) mixing a precursor with a fibrous template, (b) forming the mixture into a pre-determined shape, (c) curing the mixture to form a precursor composite, (d) carbonizing the precursor composite, and (e) decomposing the fibrous template.

Description

BACKGROUND OF THE INVENTION

[0001] Carbon fibers have a wide variety of applications. For example, U.S. Pat. Nos. 6,387,479 and 6,277,771 teach their use in composite materials reinforcement. Additionally, U.S. Pat. No. 6,037,400 teaches their use in electric wave prevention. Still further, U.S. Pat. No. 6,162,533 teaches their use in electrode construction. Other uses are also well known as described in the prior art. For example, activated carbon fibers are used as filtration media for gas separations (including removal of gas phase constituents from cigarette smoke), catalyst adsorption, treatment of waste streams or contaminated vapors, and deodorization.

[0002] Carbon articles are currently made by carbonizing precursor materials such as petroleum pitches, polyacrylonitrile, cellulose, and phenolic resins. For example, U.S. Pat. No. 4,917,835 to Lear et al. discloses a process for the production of porous shaped phenolic based carbon materials. However, poor rheological and mechanical properties of the carbon precursor materials have limited the production and processing of carbon fibers into desirable shapes. In addition, poor mechanical properties of the precursors or the resulting carbon fibers also limit the formation of suitable media for filtration applications.

[0003] Carbon is known for use in cigarette filter elements due to its ability to filter or remove constituents from mainstream smoke. In particular, activated carbon has the propensity to reduce the levels of certain gas phase components present in the mainstream smoke, resulting in a change in the organoleptic and toxicological properties of that smoke.

[0004] Examples of filter segments comprising activated carbon are described in U.S. Pat. No. 2,881,770 to Tovey; U.S. Pat. No. 3,353,543 to Sproull et al.; U.S. Pat. No. 3,101,723 to Seligman et al.; and U.S. Pat. No. 4,481,958 to Ranier et al. Certain commercially available filters have particles or granules of carbon, such as an activated carbon material, alone or dispersed within a cellulose acetate tow; other commercially available filters have carbon threads dispersed therein; while still other commercially available filters have so-called "plug-space-plug", "cavity filter" or "triple filter" designs. Examples of commercially available filters are SCS IV Dual Solid Charcoal Filter and Triple Solid Charcoal Filter from Filtrona International, Ltd.; Triple Cavity Filter from Baumgartner; and ACT from Filtrona International, Ltd. Detailed discussion of the properties and composition of cigarettes and filters is found in U.S. Pat. Nos. 5,404,890 and 5,568,819 to Gentry et al, the disclosures of which are hereby incorporated by reference.

[0005] It would be desirable to provide a cigarette filter incorporating carbon fibers and/or other materials capable of absorbing and/or adsorbing gas phase components, while providing favorable, processing, handling, absorption/adsorption, dilution and, in the case of cigarette filters, drawing characteristics, so as to be acceptable to consumers. However, no method currently exists to provide such a filter. Furthermore, commercially available activated carbons and molecular sieves are typically in granular and powdered forms. Materials in these forms do not maintain product cohesion, as granules or grains tend to settle after being packed inside a cigarette filter. It is therefore also desirable to form activated carbon fibers with improved product integrity.

SUMMARY AND DESCRIPTION OF THE INVENTION

[0006] According to an embodiment of the invention, carbon fiber and activated carbon fibers are developed with desirable cross-sectional shapes by developing their shapes from pre-formed templates.

[0007] Further according to an embodiment of the invention, shaped carbon fibers are created that have advantages in material reinforcement, electrical and other applications.

[0008] Still further according to a preferred embodiment of the invention, templated activated carbon fibers are provided with desired cross-sectional shapes that provide an efficient cigarette filter with higher TPM delivery, lower pressure drop and improved gas phase removal efficiency.

[0009] Still further according to a preferred embodiment of the invention, activated carbon fiber media are formed with controlled fiber orientation and packing density, which are critical for achieving premium performance in various applications. Preformed templates are provided with carbonaceous material and can be processed into woven or non-woven forms with desired fiber orientation and packing density. Activated carbon filtration media with controlled fiber orientation and packing density can then be formed by curing, carbonizing and activating the carbon or carbonaceous precursor fibers.

[0010] Still further according to a preferred embodiment of the invention, templated carbon fibers with controlled cross-sectional shapes provide cigarette filters that are effective at reducing main stream smoke gas phase components.

BRIEF DESCRIPTION OF THE FIGURES

[0011] Novel features and advantages of the present invention in addition to those mentioned above will become apparent to persons of ordinary skill in the art from a reading of the following detailed description in conjunction with the accompanying drawings wherein similar reference characters refer to similar parts and in which:

[0012] FIG. 1 is a side elevational view of a cigarette with portions thereof broken away to illustrate interior details including a plug-space-plug filter with a carbon filter according to the present invention;

[0013] FIG. 2 is a side elevational view of a cigarette with portions thereof broken away to illustrate interior details including a plug-space filter with a carbon filter according to the present invention;

[0014] FIG. 3 is a cross-sectional view of a template covered and impregnated with precursor;

[0015] FIG. 4 is a cross-sectional view of a template with precursor after cleaning the outside of the template;

[0016] FIG. 5 illustrates the cross-section of a trilobal shaped fibrous template according to the present invention;

[0017] FIG. 6 illustrates the cross-section of a quadrilobal shaped fibrous template according to the present invention;

[0018] FIG. 7 illustrates the cross-section of a V-shaped fibrous template according to the present invention;

[0019] FIG. 8 illustrates the cross-section of stylized I-shaped fibrous templates according to the present invention;

[0020] FIG. 9 illustrates the cross-section of a C-shaped fibrous template according to the present invention;

[0021] FIG. 10 illustrates the cross-section of an irregular shaped fibrous template according to the present invention;

[0022] FIG. 11 is a perspective view illustrating two of the four carbon fibers that remain after carbonizing the precursor and decomposing the quadrilobal template shown in FIGS. 3 and 4;

[0023] FIG. 12 is a graph illustrating the puff by puff acrolein delivery of 1R4F cigarettes and cigarettes with filters made according to the present in invention; and

[0024] FIG. 13 is a graph illustrating the puff by puff 1,3 butadiene delivery of 1R4F cigarettes and cigarettes with filters made according to the present in invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0025] Turning to the figures, a preferred embodiment of the invention will now be described.

[0026] FIG. 1 illustrates a plug-space-plug filter. Cigarette 10 comprises first plug 12, space 14, second plug 16, tobacco 18 and paper 20. The plug-space-plug filter abuts tobacco 18. The end user sets fire to paper 20 and tobacco 18 at the end opposite the filter. Air and particulate matter is then drawn toward the filter by the user. Space 14 may be filled with a material 22 such as carbon and may have voids, channels or openings 24.

[0027] Other filter arrangements are possible. For example, FIG. 2 illustrates a plug-space filter arrangement. The filter is similar to that shown in FIG. 1, but cigarette 10 instead comprises plug 12, space 14, tobacco 18 and paper 20. The plug-space filter abuts tobacco 18. Like the embodiment illustrated in FIG. 1, space 14 may be filled with a material such as carbon 22 and may have voids, channels or openings 24.

[0028] According to a preferred embodiment of the invention, templated carbon fibers are prepared by loading carbon precursor materials such as phenolic resins onto shaped fibrous templates made of low carbon yielding materials such as polypropylene that contain longitudinal channels as will be discussed in greater detail below with reference to FIGS. 3 and 4; curing the loaded carbon precursors inside the channels of the templates to form composite fibrous precursors; carbonizing the composite fibrous precursors under an inert atmosphere or in a vacuum; and decomposing the shape controlling templates to form templated carbon fibers with controlled cross-sectional shapes as will be discussed in greater detail below with reference to FIG. 11.

[0029] Fibrous template 26 may have a cross-section with a shape including, but not limited to, the shapes shown in FIGS. 5-10, which may be described as trilobal shaped, quadrilobal shaped, V-shaped, stylized I-shaped or nested stylized I-shaped, C-shaped, and irregular shaped, respectively. Templates can be shaped and formed through extrusion, spinning or other shape forming process as taught, for example, in U.S. Pat. No. 5,057,368 to Largman et al. Template 26 may be made from any polymeric material, and may leave only an insignificant amount of residue, for example, zero char yield, upon thermal decomposition. A preferred material for template 26 is polypropylene (PP). The cross-sectional shape of template 26 provides longitudinal channels 28 that may be continuous and that open to the surface of template 26.

[0030] The carbon precursor 30 may comprise solid particles, gels, foams, liquids or mixtures thereof, which yield carbon or carbonoid materials upon heating at a carbonization temperature in an inert atmosphere or under vacuum. Suitable materials in these classes include, but are not limited to, phenolic resin, petroleum pitches, polyacrylonitrile, cellulose, cellulose derivatives, polyvinyl acetate (PVA) and their mixtures. Molecular sieves, zeolites, and silicates, or other additional inorganic materials may be included in the mixture to modify the pore-distribution of the final carbonoid products. The phenolic resins proposed can be uncured or partially cured Novolak type with the presence of curing agents, or Resole (self-curing) type or mixtures thereof. In the mixture, comminuted partially cured resin as described in U.S. Pat. No. 4,917,835 to Lear et al. may be used as described or merely for binding components.

[0031] Precursor 30 is mixed with fibrous templates according to well known techniques, such as described in U.S. Pat. Nos. 6,584,979 and 5,772,768 both filed by Xue et al.

[0032] As shown in FIGS. 5-10, templates 26-26E have voids or channels 28. Precursor 30 is loaded into the channels 28 by mixing shaped templates 26-26E with precursor 30 in a container (not shown). For example, FIG. 3 illustrates loading quadrilobal template 26A with precursor 30. The template is first placed, dipped, dropped, or pulled through the container containing the precursor (not shown). Certain levels of agitation or rotation of the container may be necessary to achieve homogeneous impregnation of the channels as shown in related application Ser. No. 10/294,346, incorporated herein by reference. The weight ratio of carbon precursor to polypropylene template, also called the loading factor, is preferably within, but not limited to, the range of 0.25 and 2. When gels, suspensions or viscous liquids are used as carbon precursors, a certain amount of liquid solvent such as ethanol may be used to adjust the viscosity to allow homogeneous impregnation.

[0033] As further shown in FIG. 3, excess precursor 30 may be located outside of channel 28, which can be removed from the outside of template 26A by any well known removal method including, but not limited to, rinsing, washing in a solvent, wiping, draining or blowing. For example, the excess precursor 30 that resides outside channels 28 or on the template 26 may be removed by rubbing the filaments with a paper towel, pad, cloth or other suitable means, containing a solvent such as ethanol. After this process, precursor 30 remains within channel 28 as shown in FIG. 4.

[0034] Curing conditions may be selected so that fibrous template 26 maintains structural and/or chemical integrity while the carbon precursor 30 is cured inside the template 26 to form a non-flowing resin. The conditions may be selected based on the components in the carbon precursor, especially the uncured components used as binders. As shown, for example, in Table 1, PP templates and phenolic resin based carbon precursor can be used to practice the invention. The precursor can be cured by heating under atmosphere in a temperature from approximately 120-160.degree. C. for approximately 15-60 minutes. A certain level of acid may be added to phenolic precursors to accelerate the curing.

[0035] In the carbonizing step, the cured composite fibrous precursors can be heated in an inert environment and/or under vacuum to decompose the template and allow the carbon precursor to yield templated carbon fibers 32, as shown in FIG. 11. For example, carbonization can be achieved by heating the carbon precursor to a temperature within the range of approximately 600.degree. C. to approximately 950.degree. C. for approximately 30 minutes to four hours, though temperatures and times may be varied to achieve the desired result for any particular situation. FIG. 11 illustrates the shaped carbon fibers 32 that remain after decomposing the template illustrated in FIGS. 3 and 4. Shaped carbon fibers 32 derive their shapes from those of templates 26 and therefore may also be termed "templated carbon fibers". Portions 34 may remain in the area between the extensions of 26A that cleaning did not remove.

[0036] Table 1 lists seven examples conducted using various templates and processing conditions to achieve differing resulting channels. In the examples, carbonization can be accomplished by heating the materials under nitrogen or argon flow at a temperature of approximately 850.degree. C. for approximately one to two hours, where a phenolic-based carbon precursor and PP template are used. Carbon yields are generally in the range of 10-40% by weight depending on the PP content of the composite precursors. TABLE-US-00001 TABLE 1 Carbon Articles Processed in Accordance with the Present Invention Curing Carbonizing Process Template Cavity Loading 150.degree. C. 850.degree. C. Carbon Fiber Example Fiber ID/.mu.m Factor Min. Hour Yield % Shape OD/.mu.m 1 C-23 dpf 33-36 0.48 40 2 33 Round 43-57 2 Irregular- 15-75 0.60 15 2 23 Irregular 10 to 50 16 dpf 4 Trilobal- 26-40 0.76 15 2 22 Pentagonal 24-50 24 dpf 5 C-24 dpf 34-37 0.38 30 1 10 Round 40-60 6 C-24 dpf 34-37 0.81 30 1 17 Round 40-60 7 V-24 dpf 40-51 1.6 15 1 24 Triangle 35-40 8 V-24 dpf 40-51 1.6 15 1 23 Triangle 33-40

[0037] For the examples, a polypropylene template was mixed with a phenolic resin based carbon precursor. For examples 7 and 8, EtOH was used in phenolic precursor formulation to reduce viscosity. Templates of 16 to 24 denier per filament (dpf) were used that comprised channels with inner diameter or inner dimension (ID) of approximately 10-60 micrometers. The templates had a loading factor of between 0.38 and 1.6. Curing took place at approximately 150.degree. C. for approximately 15 to 40 minutes. A certain level of acid may be added to the phenolic precursor to accelerate this curing time. Carbonizinizing was performed at approximately 850.degree. C. for approximately 1-2 hours. Carbon yields were generally in the range of 10-24% by weight depending on the polypropylene content of the composite precursor. The carbon fibers derived their shape and outer diameter or outer dimension (OD) from the shape and ID of the template, respectively. The range given for the OD and ID reflects the pliability of the template and the characteristics of the various voids 28. For example, some of the voids had different dimensions in different directions.

[0038] It is noteworthy to point out with respect to Table 1 that the OD of some of the carbon fibers exceeds the ID. This result is obtained due to the fact that the template material was pliable and thus the precursor may have forced the ID, which was measured prior to loading, outward. Furthermore, some amount of precursor may exist between extending portions of the template that were not used in calculating the ID. For example, FIG. 11 illustrates areas 34 that were not contained within the ID of the quadrilobal surface, but that do contribute to the OD achieved.

[0039] The templated carbon fibers can be activated to form high surface area adsorptive materials for filtration applications. Many activation processes are known in the literature such as heating with CO.sub.2 or water steam. Activation can be achieved by maintaining a temperature within the range of approximately 800.degree. C. to approximately 950.degree. C. for approximately 30 minutes. For example, templated carbon fiber from Example 5 in Table 1 can be activated with CO.sub.2 at a temperature of approximately 950.degree. C. for approximately 30 minutes. At a 25% burn-off rate, a BET surface area of 1557 m.sup.2/g and a micro-pore volume (<20 .ANG.) of 0.6415 cm.sup.3/g may be obtained. These values are comparable to those of coconut based activated carbon granules, which are often used as adsorbents in cigarette filters.

[0040] Modified 1R4F cigarette models containing 66 mg and 150 mg of activated templated carbon fibers were prepared under the configurations shown in FIGS. 1 and 2, respectively. For the 66 mg model, plug 12 had a length of 12 mm, carbon article 22 had a length of 8 mm, and second plug 16 had a length of 7 mm. For the 150 mg model, plug 12 had a length of 10 mm and carbon article 22 had a length of 17 mm. The cigarettes were smoked under FTC conditions while the smoke chemistry was analyzed by FTIR and GC/MS methods. As shown in Tables 2-3 and FIGS. 12 and 13, the filters formed in accordance with the present invention are effective at reducing a wide range of smoke gas phase components when used in cigarette smoke filtration.

[0041] Table 2 compares a standard 1R4F cigarette to a cigarette containing a carbon article according to the present invention with the processing specifications described in Example 5 from Table 1. The 1R4F cigarette is a Kentucky Reference filtered cigarette provided by the Tobacco and Health Research Institute, University of Kentucky for research purposes. The first row of Table 2 lists the characteristics of control sample 1R4F, which are relatively exemplary characteristics of a control cigarette. The second and third rows of Table 2 list the characteristics of modified samples TF-66-1 and TF-66-2, respectively, which were made according to the present invention and which were provided as a percentage difference in characteristics from the control sample 1R4F. Modified samples TF-66-1 and TF-66-2 were cigarettes with the structure shown in FIG. 1 in which plug 12 was 12 mm, plug 16 was 7 mm and the carbon article 24 was 5 mm in axial length. The carbon article weighed 66 mg. However, these values are exemplary only and any lengths and/or weights could be selected.

[0042] Table 2 provides the TPM values of an 1R4F sample. The standard deviation is given with the 1R4F data. The values reported for modified samples TF-66-1 and TF-66-2 are given as a change from the 1R4F standard. A change of greater than three times the standard deviation of the 1R4F control sample is considered significant. As shown in Table 2, the acetaldehyde (M), methanol (MEOH) and isoprene (ISOP) in the total particulate matter (TPM) all decreased as a result of employing the present invention. Hydrogen cyanide (HCN) increased slightly, but not significantly. TABLE-US-00002 TABLE 2 Modified Sample Cigarettes Compared to the Control Sample cigarette. Carbon Fiber AA HCN MEOH ISOP TPM weight SAMPLE (TPM) (TPM) (TPM) (TPM) (mg) RTD (mg) 1R4F Control (TPM .times. 10.sup.-3) 51.5 9.2 6.2 23.7 13.3 140 0.0 Standard Deviation 8% 4% 98% 8% 3% 5% TF-66-1 -43% 4% -35% -54% 7.2 162 66 TF-66-2 -72% 9% -40% -62% 7.0 159 66

[0043] FIGS. 12 and 13 further illustrate how samples modified according to the present invention reduce the puff-by-puff delivery of acrolein and 1,3-butadiene. FIG. 12 illustrates puff-by-puff acrolein delivery of modified 1R4F cigarettes compared to TF-66 and TF-150 samples. FIG. 13 illustrates the puff-by-puff 1,3-butadiene Delivery of Modified 1R4F cigarettes compared to TF-66 and TF-150 samples.

[0044] For example, FIG. 12 shows the amount of acrolein in mainstream smoke for different puffs from Kentucky reference 1R4F cigarettes and the modified samples. Acrolein in cigarette smoke is measured on a per puff basis. Cigarettes are smoked with a 35 cc puff volume of two second duration, once every 60 seconds. The puff-by-puff acrolein deliveries are reported for eight determinations of 1R4F as well as the modified samples. As shown in FIG. 12, the first puff accounts for between 15 and 20% of the total delivery of the 1R4F, but generally near 0% for the modified samples. The puff process is repeated seven more times according to well known and reported methods to obtain the graph shown in FIG. 12. A similar method is used to determine the delivery of 1,3-butadiene.

[0045] As shown in FIGS. 12 and 13 the content of the constituent gases increases each puff due to saturation of the filter. However, delivery of acrolein and 1,3-butadiene is lower for the sample created using the method of the present invention. In fact, acrolein and 1,3-butadiene delivery in the samples was nearly zero for the first several puffs.

[0046] Table 3 further illustrates the benefits of the present invention. The first column lists characteristics and components common to cigarettes and cigarette smoke. The second column, labeled "1R4F Standard Deviation," lists the standard deviation of certain gas phase components present in a control 1R4F cigarette. Columns labeled TF-66 and TF-150 list the changes in component gas levels as a result of using filters made in accordance with the present invention, and more particularly Example 5 from Table 1. TABLE-US-00003 TABLE 3 Change in Gas Phase Components 1R4F Adsorbent-> Standard Runs Deviation TF-66 TF-150 Carbon Fiber/mg 66 152 Reference# 9627-798 9645-17 Gas phase components Change Change Carbon Dioxide 5% No significant change No signifi- cant change Propene 9% No significant change -60% Hydrogen Cyanide 13% -34% -83% Ethane 6% No significant change No signifi- cant change Propadiene 13% -36% -71% 1,3-Butadiene 8% -77% -97% Isoprene 5% -97% -98% Cyclopentadiene 5% -96% -98% 1,3-Cyclohexadiene 17% -100% -100% Methylcyclopentadiene 9% -100% -99% Formaldehyde 14% -95% -87% Acetaldehyde 9% -84% -97% Acrolein 14% -78% -95% Acetone 12% -100% -100% Diacetyl 5% -100% -100% Methyl ethyl ketone 4% -100% -100% Isovaleraldehyde 9% -98% -97% Benzene 8% -100% -99% Toluene 7% -100% -99% Butyronitrile 8% -100% -100% 2-Methylfuran 4% -100% -99% 2,5-Dimethylfuran 5% -100% -99% Hydrogen Sulfide 7% -67% -89% Carbonyl Sulfide 6% No significant change -38% Methyl Mercaptan 6% -71% -85% 1-Methylpyrrole 8% -100% -98% Ketene 11% -100% -93% Acetylene 13% -39% -43%

[0047] The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention, but it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings, and/or the skill or knowledge in the art of filter preparation and, more particularly cigarette filter preparation.

[0048] The embodiments described hereinabove are further intended to explain the best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.

Claims

1. A method for forming a carbon fiber comprising: mixing a carbon precursor with a fibrous template so that the carbon precursor is formed within a void created by the template shape; curing the mixture to form a precursor composite with a stable shape; carbonizing the precursor composite; and decomposing the fibrous template to yield a carbon fiber shaped by the void of the fibrous template.

2. The method according to claim 1, wherein the fibrous template comprises polypropylene.

3. The method according to claim 1, wherein carbonizing is performed in an inert media, under vacuum or a combination thereof.

4. The method according to claim 1, wherein the carbonizing step and decomposing step occur simultaneously.

5. The method according to claim 1, wherein the shaped carbon fiber has a template derived shape.

6. The method according to claim 1, wherein the carbon precursor is a phenolic resin.

7. The method according to claim 1, wherein the carbon fiber is activated by heating in the presence of C0.sub.2 or water steam.

8. The method according to claim 7, wherein activation occurs at a temperature within the range of approximately 800.degree. C. to approximately 950.degree. C. for approximately 30 minutes.

9. The method according to claim 1, wherein the outside of the template is cleaned so that precursor predominantly remains only in the void created within the cross section of the template.

10. The method according to claim 1, wherein a solvent is used to control the viscosity of the precursor during mixing.

11. The method of claim 1, wherein the template is multilobal shaped.

12. The method of claim 1, wherein the template is V-shaped.

13. The method of claim 1, wherein the template is stylized I-shaped.

14. The method of claim 1, wherein the template is C-shaped.

15. The method of claim 1, wherein the template is irregular shaped.

16. A filter comprising the carbon fiber formed according to the method of claim 1.

17. A cigarette comprising the filter of claim 16.

18. A carbon fiber formed according to the method of claim 1.

19. A smoking article comprising: a plug, a carbon filter comprising carbon fibers, and a tobacco rod.

20. A method for forming a carbon fiber comprising: mixing a fibrous template comprising polypropylene with a carbon precursor comprising phenolic resin so that the carbon precursor is formed within a void created by the template shape; cleaning the perimeter of the template; curing the mixture at a temperature of approximately 120-160.degree. C. for approximately 15 to approximately 60 minutes to form a precursor composite with a stable shape; carbonizing the precursor composite at a temperature within the range of approximately 600.degree. C. to approximately 950.degree. C.; and decomposing the fibrous template to yield a carbon fiber shaped by the void of the fibrous template.

21. The method of claim 19, further comprising activating the carbon fiber by heating the fiber in the presence of C0.sub.2 or water steam at a temperature within the range of approximately 800.degree. C. to approximately 950.degree. C. for approximately 30 minutes.