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• PITCH

  PAN (Polyacrylonitrile fibers)

PITCH based carbon fibers have lower mechanical properties and are therfore rarely used in critical structural applications.PAN based carbon fibers are under continual development and are used in composites to make materials of great strength and lightness

The raw material of PAN, acrylonitrile (AN), is a product of the chemical industry and can be manufactured as follows:

Acrylonitrile (AN) is used as a raw material in acrylic fibers, ABS resin, AS resin, synthetic rubber (NBR), acrylamide and other materials. Global production capacity is 4.67 million tons, approximately 60% of which is consumed for acrylic fibers. In the early manufacturing processes acetylene and hydrogen cyanide (HCN) were used as a raw material, whereas today nearly all AN is manufactured using what is called the Sohio process, whereby an ammoxidation reaction are applied from inexpensive propylene and ammonia. Technological advances, particularly surrounding research into improved catalysts for the Sohio process, are proceeding, promoted by a concern for energy conservation and lessening the environmental loading. The research aims include improved productivity, reduced byproducts, and lesser wastewater and waste gas.

2. Sohio process
The Sohio process was perfected in 1960 by The Standard Oil Co. of Ohio, owing to the development of an epoch-making catalyst that synthesizes AN in a single-stage reaction using propylene and ammonia. The reaction took place using the fluid-bed od. The P-Mo-Bi group is used as the catalyst and favorable fluidized conditions are maintained by adjusting the physical properties of the catalyst.

The reaction gas contains not only AN, but also acetonitrile, hydrogen cyanide and other byproduct gasses, so AN products are obtained by having the reaction gas absorbed into water, then using evaporation separation.

5. Improved processes
The Sohio process was epoch-making at the time it was developed, but improvements have been made in response to the following conditions:
(1)The AN yield of approximately 60% was not very high.
(2)The process circulated and used large amounts of water, requiring a lot of energy.
(3)Approximately 1.5 tons to 2 tons of wastewater was generated for every ton of AN produced.
(4) Treatment technology for the waste gas was incomplete.

I. Improved catalyst

II. Steam reduction
Monsanto Corp. improved the water extractive distillation stage of the Sohio process, reducing the amount of steam required to produce one ton of AN by three tons.

III. Wastewater and waste gas treatment
AN wastewater normally contains ammonium sulfate, along with small amounts of nitrile compounds, hydrocyanic acid and compounds with a high boiling point. Alkali used to be added to the wastewater before discharging, but nowadays wet oxidation processes and biological treatment processes are being employed. Bayer Inc. has developed the technology to recover high-grade ammonium sulfate from the gas generated as a byproduct of the reaction.

Polymerisation of acrylonitrile produces PAN, the most common carbon fiber feedstock

The basic unit of PAN is:

The Manufacturing Process

The conversion of PAN to carbon fibers is normally made in 4 continuous stages

Oxidation

Carbonisation(Graphitisation)

Surface treatment

Sizing

OXIDATION involves heating the fibers to around 300 deg C in air. This evolves hydrogen from the fibers and adds less volatile oxygen.

The polymer changes from a ladder to a stable ring structure, and the fiber changes colour from white though brown to black.

In this picture you see the fibre changing color.

The white PAN strands at the bottom pass through the air heated oven and begin to darken

Quite quickly they turn to black and carbon fiber is like the Ford T, As Henry said "Its any color you want, as long as it's black"

In this picture you see the "skin-core effect.

The fatter fibers are not fully oxidized and have a core, which will make a hollow low grade carbon fiber

This shows the importance of a high quality precursor of even cross section

The resulting material is a textile fiber which is fireproof, some companies actually sell this as an end product for example SGL Technic (Scotland), under the tradename PANOX. (OXidised PAN)

The temperature will determine the grade of fiber produced:

Grades of Carbon Fiber

Carbonisation Temperature (�C)	to 1000	1000 - 1500	1500 - 2000	2000 + (Graphitisation)
Grade of Carbon Fiber	Low Modulus	Standard Modulus	Intermediate Modulus	High Modulus
Modulus of Elasticity (GPa)	to 200	200 - 250	250 - 325	325 +

SURFACE TREATMENT forms chemical bonds to the carbon surface, to give a better cohesion to the resin system of the composite

SIZING is a neutral finishing agent (usually epoxy) to protect the fibers during further processing (eg prepregging) and to act as an interface to the resin system of the composite

Carbon fibers are used primarily in composites, these are structures containing two or more components, in the case of fiber reinforced composites this is the fiber and a resin. A composite containing two types of fiber, eg. carbon and glass, is known as a hybrid composite structure. The origins of textile reinforced composites are linked to the development of glass fibers, which commenced in 1938 by the Owens Corning Fiberglass Corporation (USA). Original large scale applications included air filtration, thermal and electrical insulation and the reinforcement of plastics. As the technology of textile reinforced composites expanded, a growing demand from the aerospace industry for composite materials with superior properties emeged. In particular, materials with (1) higher specific strength, (2) higher specific moduli and (3) low density were required. Other desirable properties are good fatigue resistance, and dimensional stability. Carbon fibers were developed to meet this demand.

Carbon fibers are usually mixed with resin to form a "Pre-Preg"
(Pre-impregnated)sheet, wound between release paper

A filming line used in the production of carbon fiber prepreg at SP Systems

above photos courtesy of SP Sytems

Comparison of Carbon Fiber and Steel

Material	Tensile Strength (GPa)	Tensile Modulus (GPa)	Density (g/ccm)	Specific Strength (GPa)
Standard Grade Carbon Fiber	3.5	230.0	1.75	2.00
High Tensile Steel	1.3	210.0	7.87	0.17

Carbon fibers are also unique in the range of properties that can be found, in this one generic type of material. As most carbon fiber manufactures are working in a state of constant development and improvement, the range of fibers now available to the structural engineer is always changing, look at the developments of the last 15 years:

Carbon fibers are found in the intereiors of nearly all new aircraft

And increasingly in more critical parts of the aircraft