Carbon Fiber Manufacturing from PAN and its Adhesion to Vinyl Ester Matrices
1. Background of Carbon Fibers
Carbon fiber is defined as a fiber containing at least 92 wt % carbon, while the fiber containing at least
99 wt % carbon is usually called a graphite fiber . Carbon Fiber is a relatively new and sophisticated
material that has found many uses in the field of sports, automobiles, aviation, construction and even
medical devices. It’s light and supple, yet very strong, boosting performances. Several thousand carbon
fibers are bundled together to form a tow, which may be used by itself or woven into a fabric .The
thickness varies according to use giving the object strength and flexibility suitable for its purpose.
Carbon fiber composites are generally preferred in applications where strength, stiffness, low weight,
and outstanding fatigue characteristics are essential requirements. They also can be used in operations
where high temperature, chemical inertness and high damping are demanded. They are usually
combined with other materials to form a composite. When combined with a plastic resin and wound or
molded it forms carbon fiber reinforced polymer which has a very high strength-to-weight ratio, and is
extremely rigid although somewhat brittle. However, carbon fibers are also composed with other
materials, such as with graphite to form carbon-carbon composites, which have a very high heat
tolerance . Currently, the United States of America uses nearly 60% of the world production of carbon
fibers and the Japanese account for almost 50% of the world capacity for production. The largest
producer of this fiber is Toray Industries of Japan. The world production capacity of pitch-based carbon
fiber is almost totally based in Japan .
The estimated global carbon fiber consumption is shown in Table 1 [4, 5].
Industry 1999 ( tons) 2004 ( tons) 2006 ( tons) 2008 ( tons) 2010 ( tons)
Aerospace 4000 5600 6500 7500 9800
Industrial 8100 11,400 12800 16,600 17,500
Sporting goods 4500 4900 5900 6700 6900
1,1 Polyacrylonitrile (PAN)
Organic polymers are used to manufacture carbon fiber which consist of long strands of molecules held
together by carbon atoms. A large variety of fibers called precursors are used to produce carbon fibers
of different standards and different specific characteristics. Polyacronitrile (PAN) process is the most
dominating method of manufacturing carbon fibers (about 90%). Also a small amount (about 10%) are
manufactured from rayon or the petroleum pitch process. To impart specific effects, qualities and
grades of carbon fiber, different types of liquids and gases are mixed with the precursor. The highest
grade carbon fiber with the best modulus properties are used in demanding applications such as
Figure 1. Polyacrylonitrile Polymer 
2.0 Carbon Fibers Manufacturing Process from PAN
Manufacturing Carbon Fibers by PAN process is discussed below .
Figure 2.1: Manufacturing Carbon Fibers by PAN process
Acrylonitrile plastic powder is mixed with another plastic, like methyl
acrylate and is reacted with a catalyst in polymerization process to
form a polyacrylonitrile plastic. The plastic is then spun into fibers
through tiny jets into a chemical bath. The spinning step is important
because the internal atomic structure of the fiber is formed during this
process. The fibers are later washed and stretched to the desired fiber
diameter. Stretching helps aligning the molecules within the fiber.
Fibers are heated in air to about 390-590° F for 30-120 minutes before
carbonizing to alter its chemical structure from linear to thermally
more stable ladder bonding. Hence the fibers absorb oxygen
molecules from the air and undergo rearrangement.
After stabilizing, the fibers are heated to a temperature of about
1,830-5,500° F in absence of oxygen for several minutes in a furnace.
This facilitates in formation of tightly bonded carbon crystals to each
other by expelling oxygen and other non-carbon atoms in the form of
water vapor, ammonia gas, nitrogen etc. temperatures are used to
better control the rate of heating during carbonization.
In order to improve the bonding properties with epoxies and other
materials, the carbonized fiber surface is slightly oxidized in air. The
surface is etched with nitric acid to create microspores which help in
improving mechanical bonding properties. The surface treatment
process must be carefully controlled to avoid forming tiny surface
defects, such as pits, which could cause fiber failure.
Coating materials such as epoxy, polyester, nylon are used to coat the
treated carbon fibers to protect them from any further damage due to
winding or weaving. The coated fibers are wound onto cylinders called
bobbins. The bobbins are loaded into a spinning machine and the
fibers are twisted into yarns of various sizes.
3.0 Fiber- Matrix Adhesion
For achieving desired composite properties, Fiber- Matrix adhesion is looked upon as a necessary
condition. A composite material is a combination of two or more constituent materials. The material
with better mechanical properties is usually the reinforcement while the other material is the medium in
which this reinforcement is suspended or dispersed to transfer load from reinforced fiber to fiber. The
resultant composite formed is a material whose properties are close to the properties of the
reinforcement material but in a form which can be easily fabricated into structural component. The
reinforcement materials are particulate, fiber, flake, and sheet reinforcements. Matrices may be
ceramic, metallic, polymeric and cementitious . Adhesion in fiber reinforced polymer composites is
influenced by multiple chemical and physical factors. We can observe the formation of chemical and
physical bonds at the interface where reinforcement and the matrix come in contact. Usually the surface
energy of reinforcement must be greater than that of the matrix to ensure firm adhesion. Hence
chemical groups can react with the chemical groups in the matrix to form chemical bonds such as Van de
Waals, hydrogen bonds and electrostatic bonds depending on the system .
Figure 3.0.1: Fiber-Matrix adhesion
3.1 Key factors influencing interface compatibility
Mechanical interlocking: adhesion can be improved by increasing the surface area of the fiber or by
increasing the roughness of the surface by chemical surface treatment.
Presence of defect: removal of the weakly boundary layers located at the surface of the fibers by
thermochemical surface treatment
Physical interactions (polar, dispersive): increase of the surface energy of the fiber and its polar
component due to the creation of oxycarbonated functionalities (-OH, -COOH mainly) by
thermochemical surface treatment, better wetting .
Chemical interactions: In order to increase the chemical bonding force between fiber and resin, several
kinds of organometallic coupling agent (such as titanate, zirconate, and zircoaluminate) are chosen .
4.0 Carbon Fiber Adhesion to Vinyl Ester Matrix
Free radical cured thermosetting vinyl ester resins have greater toughness and chemical resistance in
comparison to unsaturated polyester. The use of vinyl ester composites reinforced with carbon fibers
requires an improvement in the fiber-matrix adhesion levels. Vinyl ester undergo as much as 10%
volume shrinkage when cured [13, 14, and 15]. The shrinkage can develop significant instability in the
fiber/matrix interphase and hence reduces the adhesion to a great extent between the carbon fiber and
vinyl ester resin.
Vinyl esters (matrix) is a solid chunk in its raw form and is supposed to be turned into a liquid form to be
able to be usable by the composite manufacturers. So a monomer is added which is most commonly
styrene. Once in the liquid polymer resin state, the matrix and carbon fiber (reinforcement) are mixed in
the presence of a coupling agent to create a chemical reaction which cross-links the polymer molecules
thereby forming a strong reinforced composite material.
4.1 Surface Modification Strategies
Carbon Fibers have an inert and unreactive surface due to formation of the basal plane of graphite
which is very stable and unreactive. Hence, to make the carbon fiber more adhesive to matrix material,
they undergo chemical treatments to remove the native defective fiber surface leaving a structurally
sound surface for bonding .However the reaction generally occurs at the corners of these planes of the
resulting crystallites. The amount of reactive sites depends upon the fiber modulus and polymer
precursor added. For the intermediate modulus fibers used in the highest number of applications, only
20% of the fiber surface contains mostly oxygen that can be reacted with other molecules under most
conditions . However increasing the concentration of functional group, oxygen results in the failure
of the carbon fiber due to formation of pits and flaws which are greater than the critical size. Etching the
carbon fiber surface with nitric acid and other etchants help in creating micropores on the native fiber
surface formed during carbonization which can withstand higher shear loadings and hence is responsible
for greater adhesion between the matrix and the reinforcement material . Surface “finishes” are
commonly used with surface chemical treatment for carbon fibers. Finishes are usually a matrix
component applied from solvent to the fiber surface to create a layer about 0.1 microns in thickness
4.2 Role of coupling agent
The coupling agent serves as chemi-bridge between the carbon fiber and Vinyl Ester resin. The chemical
interaction between the carbon fibers and the matrix is strengthened. At the same time, the coupling
agent coating may prevent nitrogen-containing groups on the carbon fiber surface from interfering the
cationic polymerization of the matrix resin near the carbon fiber surface. For the carbon fiber with
sulfuric acid etching treatment could catalyze the Vinyl Ester ring open .
Covalent bonding, functional group reactive with –OH and COOH.
Functional group reactive with C=C
Vinyl ester monomer
Figure 4.2.1: Role of coupling agent when coating Vinyl Ester Matrices 
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