Carbon Fibers

Formation, Structure, and Properties

Leighton H. Peebles

• Publisher: CRC Press
• Year: 2018
• Print ISBN:
  9781315891323
• eBook ISBN:
  9781351087322
• Topic: Technology & Engineering
• Subtopic: Biomedical Science
• Index: Technology & Engineering

Carbon Fibers presents an up-to-date review of the progress pertaining to the formation of carbon fibers from rayon, acrylic, and pitch precursors. The book emphasizes the preparation, characterization, and properties of commercial materials. It also considers the compressive properties of carbon fibers, the lack of correlation between surface characterization and fiber-matrix interactions, and the discrepancy between surface composition as determined by XPS and the reaction of surface groups with chemical reagents. Other topics discussed include:

Chapter 1

Introduction

A number of books on carbon fibers have appeared. These include, in order of appearance, Sittig¹ (1980), Delmonte² (1981), Donnet and Bansal³ (1984), Fitzer⁴ (1985), Watt and Perov⁵ (1985), Dresselhaus et al.⁶ (1988), Figueiredo et al.⁷ (1990), and Donnet and Bansal⁸ (1990). This review covers the period 1980-1992. At the start of the project the computer program DIALOG® was used to query Chemical Abstracts for citations to carbon fibers/fibres and graphite fibers/fibres. In September 1990 there were 15,709 responses, which include patents and works on composite materials. At the time of submission to the editor in November 1993, there were more than 24,600 citations.

In earlier days, a distinction was made between fibers heat-treated in the 1000 to 1500°C range, called carbon fibers, and those heat-treated above 2000°C, called graphite fibers. As the latter are not completely graphitic, they are designated carbon fibers here. The term “graphitized” is sometimes used to distinguish fibers heat-treated above 2000°C and the term “carbonized” for those heat-treated below 2000°C. The more correct nomenclature is “high strength” fibers for the lower treatment range and “high modulus” fibers for the upper range.

Areas not covered in this review include activated carbon fibers for gas adsorption, vapor grown carbon fibers or whiskers, the formation and properties of intercalated fibers, and the isotropic pitch-based carbon fibers.

The Author takes full responsibility for all statements made in the sections at the end of each chapter entitled “Concluding Remarks.”

References

1. Sittig, M., Ed., Carbon and Graphite Fibers: Manufacture and Applications, Noyes Data Corp., Park Ridge, NJ, 1980.

2. Delmonte, J., Technology of Carbon and Graphite Fiber Composites, Van Nostrand Reinhold, New York, 1981.

3. Donnet, J.-B. and Bansal, R. C., Carbon Fibers, Marcel Dekker, New York, 1984.

4. Fitzer, E., Ed., Carbon Fibres and Their Composites, Springer-Verlag, New York, 1985.

5. Watt, W. and Perov, B. V., Eds., Handbook of Composites, Vol. 1, Strong Fibers, Elsevier, New York, 1985.

6. Dresselhaus, M. S., Dresselhaus, G., Sugihara, K., Spain, I. L., and Goldberg, H. A., Graphite Fibers and Filaments, Springer-Verlag, New York, 1988.

7. Figueiredo, J. L., Bernardo, C. A., Baker, R. T. K., and Hüttinger, K. J., Eds., Carbon Fibers, Filaments, and Composites, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1990.

8. Donnet, J.-B. and Bansal, R. C., Carbon Fibers, 2nd ed., Marcel Dekker, New York, 1990.

Chapter 2

Carbon Fibers from Rayon Precursors

I. Introduction

A number of reviews have been published concerning the conversion of rayon fibers to carbon fibers: Tang and Bacon¹ (1964), Gill² (1972), Bacon³ (1974), Riggs et al.⁴ (1982), and Konkin⁵ (1985). Because continuous filament carbon fibers based on rayon are no longer being produced commercially, this section will review briefly the manufacturing process and the chemistry involved. However, as rayon cloth continues to be converted into a carbon fiber cloth for use in two-dimensional composites, some details on the processing of fabrics will be presented.

The first public information on production of carbon fibers is due to Edison⁶ on the conversion of cellulosic fibers for use in electric lamps. The first commercial production of rayon-based carbon fibers was described by Bacon et al.⁷ Continuous filament viscose rayon can be converted into high strength, high modulus carbon fibers by an expensive process involving three separate steps: stabilization, carbonization, and high temperature heat treatment (>2800°C) under high strain conditions. The viscose rayon can also be woven into a fabric and then converted into a relatively low strength, low modulus carbon fabric.

The chemical structure of viscose rayon and suggestions on the chemical changes that occur during pyrolysis are given in Figure 1.⁸

The chemical formula in Figure 1 is (C₆H₁₀O₅)_n, which theoretically can be dehydrated to 6n C + 5n H₂O, giving a carbon yield of 44.5 wt%. Because the oxygen atoms in the main chain must be eliminated to form the pure carbon structure, CO and CO₂ are also produced during pyrolysis. A large amount of tarry substances is formed simultaneously, which must be carried away from the product to avoid sticking the filaments together.

II. Stabilization

The stabilization step, which is common to all carbon fiber precursors, is one of the most expensive and time consuming steps in production. It is a required process to fix the precursor structure so that the material will survive the succeeding high temperature heat treatments. For rayon filaments or fabric pyrolyzed in an inert atmosphere, the precursor is first dried at about 100°C and then slowly heated. During this time, the fiber shrinks and loses weight (Figure 2).

Under these conditions of heating rate and fiber choice, there is a major loss of weight in the 240 to 320°C region. The precise details will depend upon the fiber choice, its pretreatment, the heating schedule, and the atmosphere within the stabilization furnace. All processes have much less carbon yield than the theoretical, ranging from 10 to 30 wt%.

If the rayon fibers are not preconditioned for stabilization, normal practice is to heat the fiber from 100 to 400°C at 10°C/h (30 h). Properly treated fibers can then be carbonized very rapidly (on the order of a minute) after heating them to temperatures usually ranging from 1000 to 1500°C. High temperature heat treatment under tension can be done very rapidly (0.1 s or longer) at temperatures of about 2800°C.³

In commercial practice, the rayon tow is first treated with an aqueous solution of nitrogenous salts of strong acids, acids, metal halides, or various derivatives of phosphoric acid, all of which are flame retarders for cellulose. These flame retarders promote dehydration and retard tar formation. Use of these materials will reduce the stabilization time for rayon from many hours to a few minutes. Impregnation with the flame retarding materials is far more effective than stabilizing the tow in a reactive atmosphere, as the latter tends to interact with the outermost regions of the filaments, leaving the core rayon untreated.³

Figure 1 Chemical structure of viscose rayon and some suggested thermal degradation products. From Fitzer,⁸ reprinted by permission of Kluwer Academic Publishers.

III. High Temperature Heat Treatment

To obtain high strength, high modulus, rayon-based carbon fibers, the tows must be high temperature heat-treated under tension. Stretching the tows during stabilization is ineffective. Better results occur if the tow is stretched in the early phases of carbonization when the material is still plastic. The best results are obtained if the tow is stretched at temperatures around 2800°C (Figure 3).

Stretching the fibers orients the fine structure of the fibers and perfects the crystalline regions. There are definite temperature regions for each precursor type that will allow orientation and perfection, wherein molecular portions must diffuse past each other. In rayon, it is at the early stages of carbonization prior to molecular cross-linking, and again at temperatures exceeding 280O°C, wher...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Chapter 1 Introduction
• Chapter 2 Carbon Fibers from Rayon Precursors
• Chapter 3 Carbon Fibers from Acrylic Precursors
• Chapter 4 Carbon Fibers from Mesophase Pitch Precursors
• Chapter 5 Carbon Fiber Fine Structure
• Chapter 6 Carbon Fiber Properties and Structure/Property Relationships
• Chapter 7 Characterization of Fiber Surfaces and Fiber-Matrix Shear Strength
• Chapter 8 Surface Modification and its Effect on Fiber Surface and Matrix Interactions
• Chapter 9 Carbon Fiber Interactions with Nonorganic Matrices
• List of Acronyms
• List of Symbols
• Subject Index
• Materials Index