How do you make Polyacrylonitrile?
Carbon fibre is produced via 2 manufacturing processes:
1- Based on pitch (coal tar and petroleum products)
2- Based on Polyacrylonitrile (PAN)
The process involving Polyacrylonitrile will be the main focus
in this report.
· Acrylonitrile is produced commercially by the process of
propylene ammoxidation, in which propylene, ammonia and air are
reacted in a fluidized bed in the presence of a catalyst.
The steps taken in the process of carbon fibres using
Polyacrylonitrile are:
1- Polymerization:
This is the first step in the production of carbon fibre, where
the precursor (the molecular backbone of the carbon fibre)
(acrylonitrile monomer) is mixed with plasticized acrylic commoners
and a catalyst in a reactor. By continuously stirring the mixture
blending and the formation of free radicals within the
acrylonitrile's molecular structure occurs. This change leads to
polymerization which is the chemical process that creates long
chain polymers than can be formed into acrylic fibres.
2-: Spinning: This step involves Polyacrylonitrile derived from
the polymerization of acrylonitrile monomer which is mixed with
other ingredients and spun into fibres, which are washed and
stretched.
3-: Oxidation : This step involves placing the fibres into ovens
heated between 200 and 300 degrees where oxygen molecules from the
air combine with the PAN fibres and causes the polymer chains to
start crosslinking. This evolves hydrogen from the fibres and adds
less volatile oxygen which chemically alters the molecule. The
polymer changes from a ladder to a stable ring structure and it's
colour changes from white though brown to black. This stepalso
increases the fibre density from ~1.18 g/cc to as high as 1.38 g/cc
. The resulting material is a textile fibre which is fireproof.
3- Carbonization:
When the fibres are stabilized, they are heated to a temperature
of about 1,000-3,000° C for several minutes in a furnace filled
with a gas mixture that does not contain oxygen (inert atmosphere).
The lack of oxygen prevents the fibres from burning in the very
high temperatures. The gas pressure inside the furnace is kept
higher than the outside air pressure and the points where the
fibres enter and exit the furnace are sealed to keep oxygen from
entering. As the fibres are heated, they begin to lose their
non-carbon atoms, plus a few carbon atoms, in the form of various
gases. As the non-carbon atoms are ousted, the remaining carbon
atoms form tightly bonded carbon crystals and the fibre is more
than 90% carbon. The fibre loses weight and volume, contracts by 5
to 10 precent in length and shrinks in diameter.
4- Surface Treatment:
This process involves pulling the fibre through an
electrochemical or electrolytic bath that contains solutions. These
materials roughen the surface of each filament, which increases the
surface area available for interfacial fibre/matrix bonding and
adds reactive chemical groups, such as carboxylic acids.
This process is important since it helps enhance the adhesion
between matrix resin and carbon fibre. Care must be taken during
the process to avoid forming tiny surface defects, such as pits,
which could cause fibre failure.
5- Sizing:
This process involves coating the fibre with epoxy, polyester,
or nylon to protect them from damage during winding or weaving.
