ADDITIVE FOR THERMAL PRODUCTION AND REINFORCEMENT OF CARBON FIBER, AND CARBON FIBER PREPARED THERE FROM

Abstract

The present disclosure relates to an improved method for preparing carbon fiber via modified stabilization and carbonization methods during a process of forming filaments of co-polymers comprising acrylonitrile (AN) and vinyl imidazole (VIM) by adding an additive during the during the extrusion process to decrease stabilization temperature, increase crosslinking during oxidation, or decrease the temperatures of carbonization.

Claims

1. A method for formation of fibers of co-polymers comprising acrylonitrile and a vinyl imidazole co-monomer, wherein the method comprises the steps of: (a) combining acrylonitrile with the vinyl imidazole co-monomer to form a co-polymer composition; (b) melt-spinning the composition to form fibers of the composition; (c) annealing the fibers; (d) stabilizing the fibers; and (e) carbonizing the fibers, wherein an additive is added during the method and changes a characteristic of the fibers selected from the group consisting of (i) decrease stabilization temperature of the fibers by the range of from 1 C. to 50 C. as compared to the stabilization temperature of the fibers, in which the fibers would have had without the additive, (ii) increase crosslinking during oxidation that occurs during the step of stabilizing by about 5 to 50% as compared to the crosslinking during oxidation, in which the fibers would have had without the additive, (iii) decrease carbonization temperature of the fibers by the range of from 10 C. to 200 C. as compared to the carbonization temperature of the fibers, in which the fibers would have had without the additive, and (iv) combinations thereof.

2. The method of claim 1, wherein the additive is added before or during the melt-spinning step.

3. The method of claim 2, wherein the additives are added in dry blend during powderization and/or palletization.

4. The method of claim 2, wherein the additive comprises a material selected from the group consisting of peroxides, butylhydroxy toluene (BHT), or butylated hydroxyanisol (BHA).

5. The method of claim 1, wherein the additive is added after the step of melt-spinning, and before or during the step of stabilizing.

6. The method of claim 5, wherein the additive is added by fiber finishing or dip coating the melt-spun fibers with a solution comprising the additive.

7. The method of claim 5, wherein the additive is selected from the group consisting of peroxides or AIBN.

8. The method of claim 1, wherein the additive is added after the step of stabilizing, and before or during the step of carbonization.

9. The method of claim 8, wherein the additive is added by fiber finishing or dip coating the stabilized fibers.

10. The method of claim 8, wherein the additive is selected from the group consisting of BHT and BHT additives.

11. The method of claim 8, wherein the additive comprises a material selected from the group consisting of organic peroxides, butylhydroxy toluene (BHT), and butylated hydroxyanisol (BHA).

12. The method of claim 1, wherein the acrylonitrile is at least 70 wt % of the composition and the co-monomer is up to 30 wt % of the composition.

13. The method of claim 1, wherein the co-monomer is selected from 1-vinyl imidazole, 4-vinyl imidazole, 2-vinyl imidazole, and 1-methyl-2-vinyl imidazole.

14. The method of claim 1, wherein the method further comprises adding a plasticizer, and the plasticizer is an oligomer of acrylonitrile-co-methyl-1-imidazoleacrylate.

15. The method of claim 1, wherein the melt spinning is carried out at 100 C. to 200 C. in an inert atmosphere.

16. The method of claim 1, wherein the annealing is carried out at 100 C. to 150 C. under tension.

17. The method of claim 1, wherein the plasticizer is from 5 wt % to 10 wt % of the composition.

18. The method of claim 1, wherein the plasticizer is an oligomer of acrylonitrile-co-N-imidazole acrylate having a molecular weight ranging from 1,000-2,000 Daltons.

19. A carbon fiber composition made from a copolymer comprising at least 70 wt % of acrylonitrile, up to 30 wt % of a co-monomer, and an additive, wherein (a) the fibers in the carbon fiber composition are melt spun, stabilized, and carbonized and have a diameter ranging to 5-25 microns, and (b) the additive changes a characteristic of the fibers selected from the group consisting of (i) decrease stabilization temperature of the fibers by the range of from 1 C. to 50 C. as compared to the stabilization temperature of the fibers, in which the fibers would have had without the additive, (ii) increase crosslinking during oxidation that occurs during the step of stabilizing by about 5 to 50% as compared to the crosslinking during oxidation, in which the fibers would have had without the additive, (iii) decrease carbonization temperature of the fibers by the range of from 10 C. to 200 C. as compared to the carbonization temperature of the fibers, in which the fibers would have had without the additive, and (iv) combinations thereof.

20. A carbon fiber composition formed by the method of claim 1.

Description

DESCRIPTION OF DRAWINGS

[0071] FIG. 1A shows a synthesis scheme of acrylonitrile (AN) and N-vinylimidazole (VIM) to form the copolymer.

[0072] FIG. 1B shows the paths of polyacrylonitrile through stabilization (cyclization) to the final carbonized network.

[0073] FIGS. 2A-2D are diagrams of stabilizing programs describing the temperature profiles for stabilized fibers.

[0074] FIG. 3 is an optical microscopy of a stabilized AN/VIM copolymer fiber.

[0075] FIGS. 4A-4D are TGA-MS from 200 C. to 950 C. of (FIG. 4A) pure AN/VIM copolymer, (FIG. 4B) AN/VIM copolymer with AIBN, (FIG. 4C) AN/VIM copolymer with hydrogen peroxide, and (FIG. 4D) AN/VIM copolymer with benzoylperoxide additives.

[0076] FIGS. 5A-5D are diagrams of carbonization programs describing the temperature profiles for carbonized fibers.

[0077] FIGS. 6A and 6B are TGA-MS of (FIG. 5A) AN/VIM, and (FIG. 5B) AN/VIM with BHT showing a drop in the carbonization temperature down to below 800 C.

[0078] FIG. 7 is a graph showing tensile strength of carbonized fibers from base copolymer AN/VIM, the copolymer with AIBN additive, and the copolymer with hydrogen peroxide additive.

[0079] FIG. 8 is a graph showing modulus of carbonized fibers from base copolymer AN/VIM, the copolymer with AIBN additive, and the copolymer with hydrogen peroxide additive.

BEST MODE

[0080] The present invention provides an improved method for preparing carbon fiber via modified stabilization and carbonization methods. The present invention incorporate the process disclosed and taught in Yang 050 PCT Patent Application involves the use of synthesized precursors of base copolymer of acrylonitrile (AN) and N-vinylimidazole (VIM).

[0081] Improved Melt Spinning of Fibers

[0082] Yang '050 PCT Patent Application discloses and teaches various synthesis of AN-based copolymers, including the synthesis of acrylonitrile (AN) and N-vinylimidazole (VIM) to form a copolymer. See, e.g., FIG. 1 of Yang '050 PCT Patent Application. Such scheme is also shown in FIG. 1A.

[0083] For example, in one embodiment, the solution polymerization of AN and VIM and other monomers were carried out in a 250 mL flask fitted with a thermocouple probe, condenser, addition funnel and nitrogen inlet. The flask was charged with DMF and purged with nitrogen for 30 minutes. Then the monomers, AIBN and chain transfer agent, 1-dodecanethiol were added drop wise into the flask over a period of 2-5 hours. The polymerization reactions were carried out at 70 C. with continuous stirring. The polymers were precipitated in de-ionized water, filtered and washed with methanol and hexane to remove residual monomers and then dried in vacuum oven for two days till constant weight was obtained.

[0084] 82/18 ratio by weight AN/VIM copolymer precursor was ground into coarse granules in a grinder and these copolymer granules were vacuum dried at 65 C. for 3 hours. An Instron 3211 capillary rheometer was used to draw the fibers. In a typical trial, 9 g of copolymer was loaded in preheated rheometer at 180 C. under nitrogen atmosphere and left there to heat up for 10 minutes or less, after that drawing temperature was raised to 192 C.

[0085] In the present invention, the melt spinning of fibers was modified as follows: during the powderization and/or pelletization of the polymer, additives such as AIBN, peroxides, butylhydroxy toluene (BHT), or butylated hydroxyanisol (BHA), were added before extrusion was taken place. The typical drying and extrusion temperatures as disclosed and taught in Yang '050 PCT Patent Application may then be used for the fiber formation process.

[0086] Post extrusion, annealing is the process of aligning polymer chains while sequentially decreasing the fiber diameters. Typical annealing occurs at an elevated temperature from 110 C. to 130 C. Crosslinking of the fiber to impart higher strength during the stabilization process may be modified by the fiber finishing with peroxides, such as hydrogen or benzoyl peroxide. This may also be achieved by using a steam environment during annealing at these temperatures, thereby creating a high humidity during annealing.

[0087] Improved Stabilization

[0088] Stabilization is the step of cyclization of the acrylonitrile groups, as can be seen in FIG. 1B. Stabilization occurs through either the oxidative or pure cyclization routes. Both routes form a ladder polymer where the cyano functional groups will go through an interchain rearrangement.

[0089] Examples of heating programs for stabilization are shown in FIGS. 2A-2D, where the first heating program, HP1, goes directly to 300 C. at a step of 1 C. per minute, while HP2 has an original step at 150 C. for 30 minutes then to 300 C. at a step of 1 C. per minute. The program, HP2, was altered to a first step at 100 C. for an hour and half, instead of the original 30 minutes only, to allow for settling and initial oxidation of the polymer. Then the fiber was stepped up to 300 C. for 30 minutes only before cooling back down to room temperature.

[0090] It has been found that the heating program HP2 may be better for forming stabilized fibers. All fibers stabilized were performed with added tension of a weight at the end of the fiber during heating.

[0091] It has been discovered using the modified or precursor polymer with additives, the stabilization temperatures may be altered due to the additives. As FIGS. 4A-4D show, the addition of different additives may modify the stabilization temperatures of the fibers. FIGS. 4A-4D are TGA-MS from 200 C. to 950 C. of (FIG. 4A) pure AN/VIM copolymer, (FIG. 4B) AN/VIM copolymer with AIBN, (FIG. 4C) AN/VIM copolymer with hydrogen peroxide, and (FIG. 4D) AN/VIM copolymer with benzoylperoxide additives.

[0092] The typical stabilization temperature is at 300 C., while when combined with AIBN, the temperature drops to 250 C., increases with hydrogen peroxide to 350 C., then is a quicker (less time for stabilization) with benzoyl peroxide.

[0093] Improved Carbonization

[0094] Carbonization is the last step in the formation of carbon fiber, where the stabilized fiber is subjected to further heating. Examples of heating programs for carbonization are shown in FIGS. 5A-5D. Traditional programs for solution based acrylonitrile carbon fibers are shown as CP1 and CP2 (FIGS. 5A and 5B, respectively), where the temperature is ramped up to 900 C. and 1000 C. respectively. It has been seen that better results for carbonized fibers are the routes of CP3 and CP4 (FIGS. 5C and 5D, respectively) where an added isothermal step will allow for slower denitrogenation where there will be less pores and stronger fibers.

[0095] In embodiment of the present invention, the carbonization is modified by using the fibers with additives. One additive that has been found to have importance is the use of BHT (butylated hydroxytoluene). Other additives include peroxides (such as organic peroxides) and butylated hydroxyanisol.

[0096] As can be seen from FIGS. 6A-6B, the addition of BHT drops the carbonization temperature from around 1000 C., which is typical for carbon fiber, to approximately 770 C. This drop in the carbonization temperature will drop the energy expenditure for forming carbon fiber products. This also has the potential for increased strength in the fibers because the carbonization can go through full completion at lower temperatures.

[0097] FIG. 7 shows a comparison of the tensile strengths of carbonized fibers from base copolymer AN/VIM, the base copolymer with AIBN additive, and the base copolymer with hydrogen peroxide additive. FIG. 8 shows a comparison of the moduli of carbonized fibers from base copolymer AN/VIM, the base copolymer with AIBN additive, and the base copolymer with hydrogen peroxide additive.

EXAMPLES

Example 1

[0098] A copolymer consisting of 80 wt % acrylonitrile and 20 wt % 1-vinylimidazole was created via solution polymerization in dimethylformamide at a temperature of 70 C. for 18 hours. The copolymer was precipitated in water and dried for 48 hours. The resulting copolymer was extruded at a temperature of 180 C. through a 0.5-inch hole to form continuous filaments. The filaments were drawn 2 times under constant tension and heating to 120 C. The filaments were then stabilized at 250 C. under constant tension for 2 hours. The stabilized fibers were carbonized at 1000 C. for 30 minutes. The resulting carbon fibers had the following properties: tensile strength of 700 MPa, and modulus of 135 GPa.

Example 2

[0099] A copolymer consisting of 80 wt % acrylonitrile and 20 wt % 1-vinylimidazole was created via solution polymerization in dimethylformamide at a temperature of 70 C. for 18 hours. The copolymer was precipitated in water and dried for 48 hours. The resulting copolymer was extruded at a temperature of 180 C. through a 0.5-inch hole to form continuous filaments. The filaments were drawn 2 times under constant tension and heating to 120 C. The filaments were dipped in a solution of 25 volume percent of hydrogen peroxide in distilled water and dried then processed through the same method one more time. The coated filaments were then stabilized at 250 C. under constant tension for 2 hours. The stabilized fibers were carbonized at 1000 C. for 30 minutes. The resulting carbon fibers had the following properties: tensile strength of 790 MPa, and modulus of 155 GPa.

Example 3

[0100] A stabilized fiber as obtained from Example 1 was dip coated in a solution of 25 wt % of butylated hydroxytoluene in distilled water and dried then processed through the same method one more time. The coated stabilized fibers were carbonized at 1000 C. for 30 minutes. The resulting carbon fibers had the following properties: tensile strength of 958 MPa, and modulus of 175 GPa.

Example 4

[0101] A copolymer consisting of 80 wt % acrylonitrile and wt % 1-vinylimidazole was created via solution polymerization in dimethylformamide at a temperature of 70 C. for 18 hours. The copolymer was precipitated in water and dried for 48 hours. The resulting copolymer was crushed up and dry mixed with 10 wt % of azobisisobutyronitrile. The dry mixture was extruded at a temperature of 180 C. through a 0.5-inch hole to form continuous filaments. The filaments were drawn 2 times under constant tension and heating to 120 C. The drawn filaments were then stabilized at 250 C. under constant tension for 2 hours. The stabilized fibers were then carbonized at 1000 C. for 30 minutes. The resulting carbon fibers had similar properties as those of Example 2.

Example 5

[0102] A stabilized fiber as obtained from Example 4 was dip coated in a solution of 25 wt % of butylated hydroxytoluene in distilled water. The coated stabilized fibers were carbonized at 1000 C. for 30 minutes. The resulting fiber had similar properties as those of Example 3.

[0103] Modes of Additive Manufacturing

[0104] The modes in which the additive can be added into the fiber synthesis process include:

[0105] (1) Combination in dry blend during pelletization before extrusion. Melt-spun (also known as extruded) fibers will have the additives all throughout the inside and outside of the fibers.

[0106] (2) Fiber finishing, or dip coating the extruded fibers with a solution of an additive, such as the additives of peroxides or AIBN, before stabilization.

[0107] (3) Fiber finishing, or dip coating stabilized fibers with an additive solution, such as a solution of BHT or BHA additives, before carbonization.