WO2008054864A2

WO2008054864A2 - Composite material having a thermally-reactive-endcapped imide oligomer and carbon nanofillers

Info

Publication number: WO2008054864A2
Application number: PCT/US2007/067386
Authority: WO
Inventors: John W. Connell; Emilie J. Siochi; Sayata Ghose; Donavon M. Delozier; Kent A. Watson; Dennis C. Working
Original assignee: U.S.A. As Represented By The Administrator Of The National Aeronautics And Space Administration
Priority date: 2006-05-01
Filing date: 2007-04-25
Publication date: 2008-05-08
Also published as: WO2008054864A3

Abstract

Embodiments of the invention provide a composite material having sufficient strength to form structural components of an aerospace vehicle, while also having sufficient electrical conductivity to dissipate electrostatic charge. As such, a composite material comprises an imide oligomer having thermally reactive endcaps and carbon nanofillers. The imide oligomer may comprise a phenyl ethynyl terminated imide. The phenylethynyl terminated imide may have a molecular weight of less than about 5000, or may have a molecular weight in the range of about 250 to about 5000. In particular, the phenylethynyl terminated imide may have a molecular weight of about 1500. The carbon nanofillers may be selected from the group consisting of carbon nanotubes, carbon nanofibcrs, expanded graphite, exfoliated graphite and combinations thereof. The carbon nanotubes may be selected from the group consisting of single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes and combinations thereof.

Description

COMPOSITE MATERIAL HAVING A THERMALLY-REACTIVE- ENDCAPPED IMIDE OLIGOMER AND CARBON NANOFILLERS

ORIGIN OF THE INVENTION [OOOljThis invention was made in part by employees of the United States

Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefore.

CROSS-REFERENCE TO RELATED APPLICATIONS [0002]This application claims priority to U.S. Provisional Application No.

60/796,466, filed May 1 , 2006, and U.S. Non-Provisional Application No. 11/467,289, filed August 25. 2006, which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION [0003]Embodiments of the invention are directed generally to composite materials, and more particularly, to composite materiais having a thermaliy-reactive- endcapped imide oligomer and carbon nanofillers.

BACKGROUND [0004JThermally stable, high-performance resins are typically required for use in composite structures for aerospace vehicles, such as supersonic aircraft and reusable launch vehicles. Such composite structures are strong and lightweight, providing the high performance required for these types of vehicles as well as reduced fuel consumption as compared to traditional structural materials. [0005] As an aerospace vehicle moves through the atmosphere, an electrostatic charge typically builds up on the exterior of the vehicle. This electrostatic charge can interfere with and damage the vehicle's avionics such as the communication and navigation equipment. As a result, methods have been developed to dissipate the electrostatic charge. Such techniques also help protect the vehicle from lightning strikes. Among the known techniques for dissipating an electrostatic charge from an aerospace vehicle is the application of an electrically conductive coating or film to the exterior of the vehicle. However, the use of such a conductive coating or film may be problematic. Such a coating or film may be difficult to apply to the vehicle. If improperly applied, such a coating or film may not provide the necessary dissipation of electrostatic charge and may even adversely affect the aerodynamics of the vehicle. Further, such a coating or film may be easily damaged, thereby reducing or even eliminating the ability of the coating or film to dissipate the electrostatic charge. In addition to dissipation of electrostatic charge, it may also be desirable to dissipate thermal energy from an aerospace vehicle.

[0006]Accordingly, there is a need for a multifunctional material capable of providing a structural matrix of an aerospace vehicle, while possessing integrated electrical and thermal capabilities for dissipating electrostatic charge and thermal energy from an aerospace vehicle.

SUMMARY OF THE INVENTION [0007] An object of the present invention is to overcome the aforementioned drawbacks and to provide a composite material having sufficient strength to form a structural matrix of an aerospace vehicle, while also having sufficient electrical conductivity to dissipate electrostatic charge and sufficient thermal conductivity to dissipate thermal energy. [0008]In one embodiment of the invention, a composite material comprises an imide oligomer having thermally reactive endcaps and carbon nanofillers. The imide oligomer may comprise a phenylethynyl terminated imide. The phenylethynyl terminated imide may have a molecular weight of less than about 5000, such as in the range of about 250 to 5000. In particular, the phenylethynyl terminated imide may have a molecular weight of about 1500. The imide oligomer may have a glass transition temperature in excess of about 300 ⁰C.

[0009]The carbon nanofillers may be selected from the group consisting of carbon nanotubes, carbon nanofibers, expanded graphite and exfoliated graphite, as well as combinations thereof. The carbon nanotubes may be selected from the group consisting of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi- walled carbon nanotubes, as well as combinations thereof. The composite material may comprise at least about 3 weight percent of carbon nano fillers, such as comprising in the range of about 3 to about 40 weight percent of carbon nanofillers.

[OOl OJThe composite may further comprise a reinforcing material. The reinforcing material may be selected from the group consisting of organic fiber, glass fiber and carbon fiber, as well as combinations thereof.

[001 1 ]In another embodiment of the invention, a method of manufacturing a composite material comprises dry mixing carbon nanofillers and an imide oligomer having thermally reactive endcaps and then curing the mixture within the temperature range of from about 300 ⁰C to about 370 ⁰C. The method may further comprise injecting the mixture into a mold prior to curing the mixture. The mixture may be injected with a tool configured to impart shear force during the injecting, such that at least some of the carbon nanofillers are aligned in a direction of flow of the mixture.

[0012]In such a method of manufacturing a composite material, the imide oligomer may comprise a phenylethynyl terminated imide. The phenylethynyl terminated imide may have a molecular weight of less than about 5000, or may have a molecular weight in the range of about 250 to about 5000. In particular, the phenylethynyl terminated imide may have a molecular weight of about 1500. The imide oligomer may have a glass transition temperature in excess of about 300 ⁰C. Further, the carbon nanofillers may be selected from the group consisting of carbon nanotubes, carbon nanofibers. expanded graphite and exfoliated graphite, as well as combinations thereof. The carbon nanotubes may be selected from the group consisting of single- walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes, as well as combinations thereof. The composite material may comprise at least about 3 weight percent of carbon nanofillers, such as comprising in the range of about 3 to about 40 weight percent of carbon nanofillers.

[0013]The method of manufacturing a composite material may further comprise combining the mixture with a reinforcing material. The reinforcing material may be selected from the group consisting of organic fiber, glass fiber and carbon fiber, as well as combinations thereof. DETAILED DESCRIPTION OF THE INVENTION

[0014]Embodiments of the invention provide a thermally stable, structurally strong, and electrically and thermally conductive composite material that may be used for aerospace \ ehicles or any other application where such properties are desirable.

[0015]The composite material of embodiments of the invention comprises a controlled molecular weight imide oligomer having thermally reactive endcaps or terminations. Embodiments of the invention will be described herein comprising a phenylethynyl-terminated imide (PETI), The structure of phenyl ethynyl is:

However, other thermally-reactive terminations of the imide oligomer may be used,

including but not limited to maleimide O , phenylmaleimide

nadimide.

[0016]ϊn embodiments in which the imide oligomer is terminated by phenylethynyl. the phenylethynyl terminated imide will typically have a glass transition temperature (Tg) in excess of about 300 ⁰C. An example of such a phenylethynyl terminated imide is PETI-330. PETI-330 (U.S. Patent No. 6,359,107) is a low molecular weight imide oligomer that has a low and stable melt viscosity at about 280 ⁰C and a glass transition temperature of about 330 ⁰C after curing at about 370 ⁰C. [0017]The phenylethynyl terminated imide of embodiments of the invention may have a controlled molecular weight, typically in the range of about 250 to about 5000. In particular, the phenylethynyl terminated imide of embodiments of the invention may have a molecular weight of about 1500.

[0018] As is discussed in more detail below, the thermally-reactive terminations and the controlled molecular weight enable the composite material to remain workable and non-brittle during processing into the desired shapes, as the material would typically be maintained during processing at a temperature well below Tg. This temperature control during processing enables the molecular chains of the imide oligomer to remain relatively short and non-crosslinked.

[0019]The composite material of embodiments of the invention further comprises carbon nanofillers, such as carbon nanotubes, carbon nanofibers, expanded graphite and exfoliated graphite, as well as combinations thereof. A carbon nanotube is a carbon allotrope having a cyiindrical configuration. Carbon nanotubes may be single-walled (SWCNT), double- walled (DWCNT), or multi-walled (MWCNT). Carbon nanofibers (CNFs) have a wide range of morphology, from disordered bamboo-like structures to highly graphitized ^iCcup-stacked" structures where conical shells are nested within one another. Graphite typically exists in a large number of adjacent layers or sheets that are difficult to separate. Exfoliation is the process of separating the graphite sheets into a smaller number of adjacent layers with larger spacing between layers or even into single layers.

[0020]Embodiments of the invention typically comprise in the range of about 3 to about 40 weight percent of carbon nanofillers. Such a range provides sufficient electrical conductivity to effectively dissipate electrostatic charge and sufficient thermal conductivity to effectively dissipate thermal energy without adversely affecting the physical properties of the material (e.g., without causing brittleness). The electrical conductivity of the material also enables the material to function as a sensor.

[002 l]The composite may further comprise a reinforcing material, such as organic fiber, glass fiber or carbon fiber, or combinations thereof. The reinforcing material may be added as large, loose fibers or may be in the form of a fabric that is subsequently impregnated with the imide oligomer/carbon nanofiller mixture. This reinforcing material provides additional strength to the composite material, enabling the material to be used, for example, for the structural elements of aerospace vehicles or other load-bearing or stress-bearing structures. [0022]The composite material of embodiments of the invention may be manufactured by dry mixing the carbon nanofillers (e.g., carbon nanotubes, carbon nanofibers, expanded graphite or exfoliated graphite, or combinations thereof) and the thermally

terminated imide. As described above, in the range of about 3 to about 40 weight percent of the carbon nanofillers may be combined with the thermally reactive terminated imide. Also as described above, the mixture would typically be maintained during processing at a temperature well below the Tg of the thermally reactive terminated imide. The carbon nanofillers and the thermally reactive terminated imide may be dry mixed using, e.g., ball-milling, to obtain a powder-like consistency of the mixture. After mixing, the grinding media will typically be separated from the mixture using a sieve.

[0023 jThe mixture may then be injected into a mold to form the material into the desired shape. Because the temperature of the mixture has been maintained during processing at a temperature well below the Tg of the thermally reactive terminated imide, the mixture is easily processed using standard molding techniques. The mixture may be injected with a tool configured to impart shear force during the injecting process (e.g., by using tapered injection ports), causing at least some of the carbon nanofillers to align in the direction of flow. It may be necessary to de-gas the mixture to remove moisture and air from the mixture prior to injecting, such as by heating the mixture to a predetermined elevated temperature and holding the mixture at that temperature for approximately one hour. The mixture may be injected at, e.g., a predetermined elevated temperature and an injection rate of up to about 200 cubic centimeters per minute. The mold is typically clamped in a press to assure adequate sealing.

[0024]The mixture may then be cured by raising the temperature, in the example of PETI-330, to approximately 370 ⁰C and maintaining that temperature for approximately one hour. Because of the thermally reactive terminations, the curing lengthens and crosslinks the molecular chains of the imide oligomer, thereby imparting strength and toughness to the composite material.

[0025]If desired, the mixture may be injected into a mold containing a reinforcing material, such as organic fiber, glass fiber or carbon fiber, or combinations thereof, to create a fiber composite material having the advantageous properties (e.g., electrical and thermal conductivity) provided by embodiments of the invention while maintaining the known advantageous properties (e.g., structural strength) of fiber composite materials. The fibers of the reinforcement may be arranged in a variety of patterns such as unidirectional. woven, random, and three dimensional stitched, and may comprise continuous or discontinuous fibers.

Example I: Preparation of PETI-330 with various % (wt/wfl of MWCNT

[0026]Dry mixing of PETI-330 was carried out in alumina-fortified porcelain jars using cylindricaliy shaped zirconia grinding media (9.5 mm X 9.5 mm). The jars were placed on rollers and rotated for 48 hours at 120 rotations per minute. Initially, the mixing was performed on a small scale using jars with a capacity of 0.4 liters. Mixtures containing 3, 5, 7, 10, 12, 15, 20, and 25 weight percent (wt %) MWCNT were prepared. Based on the melt rheology and other characterization, two samples were chosen for scale-up using a 1.9 liter jar. After mixing, the grinding media was separated from the powder using a sieve and the powder was collected. Typical recovery yields ranged from 70-80% of the total weight due to adherence of the powder to the walls of the jar and to the grinding media.

[0027]The obtained samples were molded using a molding tool constructed of INVAR, an al!o^v of steel and nickel. The infection ⁿorts of the tool were tapered to provide shear during injection to orient at least some of the MWCNTs. Approximately 300 grams of the PETI-330/MWCNT sample was de-gassed in the injector by heating to 280 ⁰C and held at this temperature for 1 hour prior to injection. The de-gassing step is generally required in resin transfer molding to remove moisture and air from the resin. After the sample was injected at 288 ⁰C at a rate of 200 cubic centimeters per minute, the tool was clamped in a press to assure adequate sealing and held at a minimum of 1.38 megapascal of hydrostatic pressure, heated to 371 ⁰C and held at this temperature for 1 hour. The plaques were cooled in the mold.

[0028JAs shown in Table 1 below, the Tg of the neat PETI-330 was 326 °C and only a small change in Tg was observed with the addition of MWCNTs, with the Tg reaching 332 ⁰C when 20 wt % MWCNTs were added. The thermal degradation data from the thermogravimetric analysis, also shown in Table 1 , showed that the temperature at 5 wt % loss decreased as the MWCNT content increased. The neat resin lost 5 wt % at 508 ⁰C whereas the sample containing 25 wt % MWCNT shows a lower thermal stability, losing 5 wt % at 462 ⁰C. Table 1 : Tg and Thermal Degradation (Example I)

[0029]Hardness is the property of a material that enables the material to resist plastic deformation, usually by penetration. The hardness testing of plastics is most commonly measured by the Rockwell hardness test of the Shore (Duro meter) hardness test. Both methods measure the resistance of the plastic to indentation. Both scales provide an empirical hardness value that does not correlate to other properties or fundamental characteristics. Both the Rockwell and Shore D hardness of molded samples were determined, as shown in Table 2 below. While the hardness of the samples decreased with the addition of MWCNTs, the decrease was not large enough to prohibit the use of such PETI-330/MWCNT composite materials in aerospace vehicles. Table 2: Hardness (Example I)

[0030]Surface and volume resistivities of molded samples were measured, as shown in Table 3 below, according to ASTM D-257. The molded sample of the neat resin was electrically insulating, however the resistivity decreased with an increase in the concentration of MWCNTs. Typically, for electrostatic charge dissipation to occur, the surface resistivity should be between 10⁶ and 10^lϋ Ω/sq. With the addition of MWCNTs, the molded samples were sufficiently conductive to dissipate electrostatic charge.

Table 3: Resistivity (Example I)

[0031 ] Thermal conductivity was measured on molded samples using a NETZSCH* LFA 447 NANOFLASH^® according to ASTM E-1461. Table 4 shows the thermal conductivity of neat and MWNT-filled samples. With a loading of 10 wt %, conductivity was improved approximately 3-fold. Improvement of approximately 4-fold was seen with 15 wt % loading. This indicates that the nanotubes are forming an APPL1CAT1ON

effective network in the polymer matrix which sufficiently conducts heat.

Table 4: Thermal Conductivity (Example I)

[0032] Additional discussion of Example 1 is provided in Sayata Ghose, et. al, "Preparation and Characterization of PETI-330/Multi- Walled Carbon Nanotube Composites," SAMPE 2005 Technical Conference Proceedings: Advancing Materials in the Global Economy - Applications, Emerging Markets and Evolving Technologies, Eong beach, CA, May 105, 2005, herein incorporated by reference in its entirety.

Example 11: Preparation of PETI-330 with various % (wt/wf) of CNF

[0033]Dry mixing of PETI-330 and CNFs (PYROGRAF^®-III obtained from Applied Sciences, Inc.) was carried out in alumina-fortified porcelain jars using cylindrically shaped zirconia grinding media (9.5 mm X 9.5 mm). The CNFs had diameters between approximately 70 nm and approximately 200 nm and a length between approximately 50μm and approximately 100 μm. The jars were placed on rollers and rotated for 48 hours at 120 rotations per minute. Initially, the mixing was performed on a small scale using jars with a capacity of 1.9 liters. Mixtures containing 10, 30, 40 and 50 weight percent (wt %) CNFs were prepared. Based on the melt rheology and other characterization, two samples were chosen for scale-up using a 5.7 liter jar. After mixing, the grinding media was separated from the powder using a sieve and the powder was collected. Typical recovery yield was approximately 90% of the total weight due to adherence of the powder to the walls of the jar and to the grinding media. APPLICATION

[0034]The obtained samples were molded using an INVAR^® molding tool. The injection ports of the tool were tapered to provide shear during injection. Approximately 300 grams of the PETI-33O/CNF sample was de-gassed in the injector by heating to 280 ⁰C and held at this temperature for 1 hour prior to injection. After the sample was injected at 288 ⁰C at a rate of 200 milliliters per minute, the tool was clamped in a press to assure adequate sealing and held at a minimum of 1.38 megapascal of hydrostatic pressure, heated to 371 ⁰C and held at this temperature for 1 hour. The plaques were cooled in the mold.

[0035JAs shown in Table 5 below, the Tg of the neat PETI-330 was 326 ⁰C and only a small change in Tg was observed with the addition of CNFs, with the Tg reaching 332 ⁰C when 50 wt % CNFs were added. Both Rockwell and Shore D hardness of the molded samples were determined as provided in Table 6. Surface and volume resistivities of molded samples were measured, as shown in Table 7, according to ASTM D-257. Thermal conductivity was measured on molded samples using a NETZSCH" LFA 447 NANOFLASH ^l! according to ASTM E- 1461. Table 8 ihυws the thermal conductivity of neat and CNF-filled samples.

Table 5: Tg (Example II)

APPLICATION

Table 6: Hardness (Example II)

Table 7: Resistivity (Example II)

Table 8: Thermal Conductivity (Example II)

[0036J Additional discussion of Example II is provided in Sayata Ghose, et. al, "Fabrication and Characterization of High Temperature Resin/Carbon Nanofϊber Composites,^" High Performance Polymers, 18 (4), 527 (2006).

[0037]Further discussion of the present invention is provided in Sayata Ghose, et. al, "Incorporation of multi-walled carbon nanotubes into high temperature resin using dry mixing techniques," Composites. Part A: Applied Science and Manufacturing, 37(3): 465, 2006; Sayata Ghose, et. al, "High temperature resin/carbon nanotube composite fabrication.¹" Composites Science and Technology, 66(13): 1995, 2006; Ghose Sayata, et. al, "Fabrication and Characterization of PETI-330/Multi- Walled Carbon Nanotube Composites," Proceedings of the Fifth World Congress Nanocomposites 2005, Commercial Applications, Business Opportunities, Scientific Discoveries, San Francisco. CA, August 22-24, 2005; and Sayata Ghose, et. al, "Fabrication and Characterization of High Temperature Resin/Carbon Nanofiller Composites," Multifunctional Nanocomposite International Conference, Honolulu, HI, Sept 20-22, 2006, all herein incorporated by reference in their entirety.

[0038] As can be seen from the foregoing, the composite material of embodiments of the invention provides physical and electrical cliaiacteiistics which are desirable for use in aerospace vehicles, among other applications.

[O039]While the invention has been described with reference to specific embodiments thereof, it will be understood that numerous variations, modifications and additional embodiments are possible, and all such variations, modifications, and embodiments are to be regarded as being within the spirit and scope of the invention.

[0040]What is claimed as new and desired to be secured is:

Claims

1 . A composite material comprising: an imide oligomer having thermally reactive endcaps; and carbon nanofϊllers.

2. The composite material of claim 1. wherein the imide oligomer comprises a phenylethynyl terminated imide.

3. The composite material of claim 2, wherein the phenylethynyl terminated imide has a molecular weight of about 1500.

4. The composite material of claim 2, wherein the phenylethynyl terminated imide has a molecular weight of less than about 5000.

5 The composite material of claim 2. wherein the phenylethynyl terminated imide has a molecular weight in the range of about 250 to about 5000.

6. The composite material of claim 2, wherein the carbon nanofϊllers are selected from the group consisting of carbon nanotubes, carbon nanofibers, expanded graphite, exfoliated graphite and combinations thereof.

7. The composite material of claim 6, wherein the carbon nanotubes are selected from the group consisting of single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes and combinations thereof.

8. The composite material of claim 1, wherein the material comprises at least about 3 weight percent of carbon nanofillers.

9. The composite material of claim 8, wherein the material comprises in the range of about 3 to about 40 weight percent of carbon nanofillers.

10. The composite material of claim 1, wherein the imide oligomer has a glass transition temperature in excess of about 300 ⁰C.

1 1 . The composite material of claim 1 , further comprising: a reinforcing material.

12. The composite material of claim 1 1 , wherein the reinforcing material is selected from the group consisting of organic fiber, glass fiber, carbon fiber and combinations thereof.

13. A method of manufacturing a composite material, the method comprising: dry mixing carbon nanofillers and an imide oligomer having thermally reactive endcaps; and curing the mixture at a temperature in the range of about 300 ⁰C to about 370 ⁰C.

14. The method of claim 13, further comprising: injecting the mixture into a mold prior to curing the mixture.

15. The method of claim 14, wherein injecting the mixture comprises injecting the mixture with a tool configured to impart shear force during the injecting, such that at least some of the carbon nanofillers are aligned in a direction of flow of the mixture.

16. The method of claim 13, wherein the imide oligomer comprises a phenylethynyl terminated imide.

17. The method of claim 16, wherein the phenylethynyl terminated imide has a molecular weight of about 1500.

18. The method of claim 16, wherein the phenylethynyl terminated imide has a molecular weight of less than about 5000.

19. The method of claim 16, wherein the phenylethynyl terminated imide has a molecular weight in the range of about 250 to about 5000.

20. The method of claim 13, wherein the carbon nanofillers are selected from the group consisting of carbon nanotubes, carbon nanofibers, expanded graphite, exfoliated graphite and combinations thereof.

21. The method of claim 20, wherein the carbon nanotubes are selected from the group consisting of single-walled carbon nanotubes, double-walled carbon nanotubes, multi-walled carbon nanotubes and combinations thereof.

22. The method of claim 13, wherein the mixture comprises at least about 3 weight percent of carbon nanofiilers.

23. The method of claim 13, wherein the mixture comprises in the range of about 3 to about 40 weight percent of carbon nanofillers.

24. The method of claim 13, wherein the imide oligomer has a glass transition temperature in excess of about 300 ⁰C.

25. The method of claim 13, further comprising: combining the mixture with a reinforcing material.

26. The method of claim 25, wherein the reinforcing material is selected from the group consisting of organic fiber, glass fiber, carbon fiber and combinations thereof.