Carbon fibre formation and use

Carbon fibre

The term carbon fibre is referred to those fibres that have been heated to temperatures up to 1500°C a contains up to 95% of elemental carbon, Another type of carbon fibre is commonly known as 'graphite' fibre. Graphite fibres are those which have been heated to higher temperatures, often above 2500°C, and are about 99% carbon. Such fibres have broader graphite-like layers, which are closely packed in a parallel alignment.

carbon fibre

Carbon fibre formation

Formation of carbon fibre generally consists of the following stages:
(a) Conversion of the precursor material into a fibre, if it is not in fibre form.
(b) Elimination of all chemicals other than carbon by thermal cleavage, oxidation, etc.
(c) Carbonization for conversion into carbon fibre.
(d) Graphitisation for conversion into graphite fibre.

Precursor Material
Carbon fibres are generally manufactured by pyrolysis and thermal treatment of organic precursor fibres like rayon, polyacrylonitrile, pitch, coal tar. Several temporally stable polymers and/or fibres can be used as a precursor material for conversion into carbon fibre. These fibre include poly (vinyl alcohol), polyimide, aromatic polyamide, and polybenzimidazole. The main characteristics of the precursor material for the conversion are that the melting temperature should be substantially higher than the decomposition temperature.

Carbon fibre from rayon
Cellulose or rayon is one of the most widely used precursors or starting material for making carbon fibre. Rayon yields 15-30% by weight of carbon and does not melt during decomposition. So the physical form of the starting material can be maintained. The conversion generally takes place in the furnace at different temperatures and at different heating rates. The heating takes place in different stages like :

(a) In the first stage of heating, the temperature should rise from 10°C/hr to 50°C/hr in the temperature range 100° - 400°C. The temperature range 250° - 300°C is very critical as the maximum weight loss takes place in this region.
(b) In the second stage of heating, the temperature should rise from 50°C/hr to 100 C/hr in the range 400 - 900°C.
(c) In the third stage of heating, the fibre is heated to 3000°C until graphitization takes place.

The multistage mechanism for the conversion of cellulose to carbon in outline is as follows:

Stage II: Physical desorption of water in the temperature range of 25°. 150°C.

Stage Il : Dehydration from the cellulose unit at 150°C- 240°C

Stage III : Thermal cleavage of the cyclotide linkage and scission of other C-O bonds and to some C-C bonds in the temperature range of 240°C - 400°C via a free radical reaction.

Stage IV : Aromatization at or above 400°C.

Carbon fibre from pan
Polyacrylonitrile (PAN) is used as a starting material to manufacture carbon fibre. The denier of PAN fibre should be 1 to 3. Different stages to convert PAN fiber to carbon fibre are as follows: Oxidation The fibre is oxidized at 200 - 250°C in air for sufficient time. The fire may be kept in stretch/tension conditions during oxidation. After oxidation, the fibre is black and has a shiny appearance. Carbonizations The fibre is further heat-treated in an inert condition and in the temperature range of 800°C to 1000°C for at least one I hr.
The fibre must be kept out of contact with air for which nitrogen gas should be used during the reaction. Heat treatment or graphitization The fibre is further heat-treated in the temperature region of 1100°C - 2500 C. Heat treatment at 1100°C- 1500°C would yield a lower modulus but high strength fibre. If the fibre is stretched during the oxidation or carbonization stage, the fibre will be an ultra high strength fibre.

Graphitization at 2800°C would yield a graphite fibre, which is an oriented high modulus fibre. The various mechanisms for the conversion of PAN to carbon fiber are listed below: Oxidation induces a chemical reaction with the formation of oxygen bridge - linking two PAN molecules and water is eliminated. Stage I: Stage II: Carbonisation will form a carbon ring structure by carbonizing oxidized PAN fibre with the elimination of water and hydrocyanic acid. Stage III: Further heat treatment or graphitization modifies the structure of the fibre to an oriented fibrillar structure.

Carbon fibre from the pitch
Petroleum and coal-derived pitches are the basic raw materials for the manufacture of carbon and graphite fibre. Pitch is a complex mixture of aromatic hydrocarbon molecules of wide molecular weight distribution. It. contains more than 90% carbon, much more than rayon or PAN.
Initially. low-modulus carbon fibre from the pitch was manufactured. In this process. pitch is melted and the thermoset by heating in o7one and/or air. This fibre is basically a low modulus fibre.

High modulus carbon fibre from the pitch is manufactured by converting the pitch to a mesophase or liquid crystal pitch. This mesophase pitch can be melt spun and drawn into fine filaments with high orientation. The conversion of the pitch into high modulus carbon fibre consists of the following stage

a) Polycondensation of the pitch for 2-8 hrs at 350°C - 450°C or hydrogenated with selected chemicals at 360°C - 430°C for 2-6 hrs.
b) Purification and heat treatment of pitch either for 10-15m at 500°C or for me hr to 8 hr at 350°C - 450°C.
c) Melt spinning into fibre at 290°C - 330°C.
d) Oxidation for 20 m to I hr at 250°C - 600C.
e) Carbonization for 10m - 30m at 1400°C - 1500°C. 
f) Graphitisation for 5 min at 2500°C - 2700°C to manufacture graphite fibre.

Properties of carbon fibre

The physical properties of different types of carbon fibre are shown in Table 17.5. The fibre does not melt. It oxidizes very slowly in the air at temperatures above 330°C. The fiber exhibits excellent resistance to acids, alkalies even at high concentrations and temperatures. It is also inert to all solvents. However, strong oxidizing agents will degrade carbon fibre. Also, the fiber has poor resistance to hydro chlorites. The fibre content is dyed. Physical properties of different carbon fibre

Properties of carbon fibre

Application of carbon fibre

Carbon fibre was first used as a light bulb filament in 1879. Diversifications in its application started in 1944. From 1944 to 1964 low modulus carbon fibre was used as thermal insulating material as well as in electrical insulation. After the invention of high strength and high modulus carbon fibres from PAN and pitch, the fibre is used in high-performance composites in particular as rigid lightweight and dimensional stable reinforced material for aircraft and rockets. The fiber is marketed as Celion, Hi-Tex, and Thrnel. 
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