Recent Developments on Carbon Fibers from Rayon-Based Precursors

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1 Recent Developments on Carbon Fibers from Rayon-Based Precursors Gajanan Bhat, Sammy Akato, Nicholas Cross, The University of Tennessee, Knoxville West Hoffman, AFRL, Edwards, AFB, CA and Farhad Mohammadi, Advanced Cerametrics, Lambertville, NJ

2 Carbon Fiber Precursors Rayon (or regenerated cellulose) PAN (Polyacrylonitrile) Pitch (petroleum and coal tar based) Natural materials such as lignin, wool, cotton, ramie, and jute High performance highly crystalline aramid fibers such as Kevlar Polymeric materials like phenolic resins

3 Rayon Fibers Rayon is produced from naturally occurring cellulose polymers Some properties of rayon fibers Availability Low cost Non-melting character Ease of production Repeat unit of cellulose ~ 44% carbon content

4 Earlier Research on Rayon-Based CF Thomas Edison used CCF as filaments for incandescent lamps (1880) Bacon et al. patented a process using viscose rayo fibers to produce high strength, high modulus RCF Union Carbide used stress graphitization to produce strong RCF (1959) RCF were the first to be qualified by NASA Later other precursors became more prominent

5 Why This Study? Currently rayon fibers are not produced in the US Rayon-based CF are still of interest to DoD Experimental rayon fibers (Advance Cerametrics) are being evaluated as a candidate for CF precursor

6 Precursors Commercial rayon fibers from Lenzing, Austria Experimental rayon fibers from Advanced Cerametrics, Lambertville, NJ USA X-ray patterns of the commercial rayon and experimental rayon fibers

7 Physical and Mechanical Properties of Rayon Fibers

8 Production of Carbon Fibers Stabilization of the precursor at low temperatures Oxidation in air at lower temperature (<400 C) Carbonization in an inert atmosphere up to ~1500 C Longitudinal orientation Development of the crystalline ordering Graphitization in an inert environment up to 3000 C Optional for high modulus

9 Temperature (C) Set-up for Stabilization and Carbonization C 600C 1200C

10 Production of Rayon Carbon Fibers (RCF) Pretreatment Impregnated in 1 normal solution of phosphoric acid for 5 hours Dried at room temperature overnight Oxidation in air at lower temperature up to 380 C Carbonization in an inert atmosphere (Nitrogen) up to 1200 C under tension HTT

11 Effect of HPA Pretreatment DSC TGA Catalyzed the dehydration reactions Lowered degradation temperature over a wider range Suppressed the release of volatile organic substances by reacting with hydroxyl groups

12 Stabilization Conditions

13 Carbonization Conditions All samples were stabilized first from 110 C to 380 C in air for a total time of 3 hours Applied load for sample 6003 is 10g and applied load for sample 6005 is 50g

14 SEM of Untreated and Treated rayon Fibers after Pyrolysis

15 TGA TGA Scans of Fibers

16 Change in Crystalline Structure Precursor Theta The dehydration only changed the crystal structure partially Fibers become amorphous at 300 C Order develops after 380 C

17 SEM SEM micrographs of the surface and cross-section of the carbonized sa The fiber diameter changes with temperature Smaller fibers Smoother surface The fibers appear to be of good quality

18 % Content Elemental Analysis Carbon content Hidrogen content Precursor Stabilized Sample ID

19 EDAX EDXA results of the precursor, the stabilized fibers and the carbonized fibers to 1200C respectively

20 Mechanical Properties

21 Effect of tension during carbonization

22 High temperature heat treatment HT to 1700 C at ORNL

23 Change in Thermal Behavior (TGA)

24 % content Elemental Analysis Carbon content Hidrogen content Precursor Sample ID

25 SEM of CF SEM micrographs at 2KX of the carbonized sample

26 Comparison of the Produced CFs

27 SEM of HTT CF

28 Initial Observations The use of phosphoric acid shifts the pyrolysis reactions to lower temperatures As the pyrolysis progresses, the structure changes The mechanical properties of the obtained fibers need to be improved To improve the mechanical properties of the obtained fibers: Lower fiber diameter Control the shrinkage Higher HTT

29 Studies with a Smaller Diameter Precursor Precursor Diameter ~ 12 micron

30 Shrinkage and Fiber Diameter Change

31 Shrinkage (%) Shrinkage 20 Shrinkage Percentages with Varying Tension nc-17 (10 grams) nc-16 (20 grams) nc-14 (50 grams) nc-18 (75 grams) nc-19 (100 grams) nc-110 (120 grams) nc-30 (50 grams) Temperature ( C)

32 Amount of Shrinkage (%) Shrinkage grams 120 grams Temperature (Celsius)

33 SEM of Carbon Fibers

34 Elemental Analysis

35 Future Work Smaller diameter fiber stabilization optimization Tension is critical Tension during Carbonization & HTT is also important Increase HTT to achieve higher C content Continuous processing

36 Acknowledgements AFRL Advanced Cerametrics Lenzing Harper International