Carbon Fibers from PAN based precursors

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PAN based precursors
Introduction to PAN based Carbon Fiber A PAN based carbon fiber is produced by oxidative stabilization of a PAN precursor (performed in a multi-zone oven) followed by a two-stage carbonization process (performed in a Harper LT / HT furnace), with an additional ultra-high temperature heat treatment stage (performed in a Harper UHT) to manufacture a high modulus carbon fiber. Briefly, the process involves the removal in gaseous of all elements other than carbon in the PAN based filaments and then carbonizing it to form as much graphitic carbon (sp2) as desired.
PAN Precursors Amongst the many types of precursors used for the production of carbon fibers, polyacrylonitrile (PAN) has been the most popular. One of the chief attractions of PAN (CH2=CHCN) is that the polymer has a continuous carbon backbone and the nitrile groups are ideally replaced for cyclization to occur producing a ladder polymer, believed to be the first stage towards the carbon structure of the final fiber. The carbon content of PAN is 68% and it is not surprising that PAN precursors have a carbon yield of around 55%, coupled with the ability to produce high modulus fibers. As can be seen in the table below, only pitch based precursors have higher carbon content however due to the predominantly graphitic nature of the carbon in pitches, the compressive strength of pitch based carbon fibers is extremely poor relative to PAN based carbon fibers. Precursor Carbon Content PAN 68 Cellulosic Precursor 45 Pitch 85
PAN belongs to a family of acrylic precursors, which were developed by companies that were established commercial producers of textile grade acrylic fibers. The companies that have adapted to acrylic polymer formulation suitable for production of carbon fibers have morphed into the leading players in this arena. Further, several leading carbon fiber companies have their own precursors and their compositions are proprietary knowledge which is kept a tightly guarded secret.	Precursor	Carbon Content
	PAN	68
	Cellulosic Precusor	45
	Pitch	85

Important features that distinguish a PAN precursor that can be processed into carbon fibers: • A polymer with an acrylonitrile content > 85% is essential to produce carbon fibers. • A relative molecular weight of about 100,000 is needed for a precursor fiber to yield carbon fibers with good mechanical properties. • The technical and commercial efficacy of the polymer spinning process depends upon the polymer dope concentration in the spinning solvent and the temperature controls the spinning and the final shape of the spun filament. • The construction and cleanliness of the spinning equipment (spinneret) had a large bearing on the final cosmetics of spun precursor filaments. • The spun filament should have a relatively fine count (or diameter), which has a bearing on the chemical reaction rate and the production rate. A larger diameter of the spun precursor filament would decrease the reaction rates and increase mass rates; and on the other hand a smaller diameter would promote a uniform reaction rate and decrease production rates. A suitable dTex is chosen to balance both reaction rates and mass flow rates, and is usually around 1.00 – 1.22 dTex. PAN homopolymer PAN homopolymer is not an easy polymer to process into carbon fiber, as it oxidizes very sharply and thereby the oxidative stabilization reaction stage becomes difficult to control due to sudden and rapid evolution of heat, coupled with a relatively high initiation temperature. The rapid evolution of heat can lead to chain scission leading to poor fiber properties. This exothermic reaction can be controlled by suitable comonomers such as itaconic acid (ITA), methacrylate (MA); and various other vinyl esters, vinyl amides, vinyl halides, ammonium salts of vinyl compounds and various others. The co-monomers also lower the glass transition temperature which in turn affect the shrinkage behavior of precursor fibers in the oxidative stabilization reaction. Spinning of PAN Fibers PAN copolymer (~ 95% PAN homopolymer) is dissolved in a solvent such as dimethylformamide, dimethylacetamide, sodium thiocyanate or zinc chloride to form a highly concentrated polymeric solution and is pumped through a spinneret comprising of several fine capillary holes into a coagulation bath that extracts the solvent from the fiber. This stage of solvent extraction has a large bearing on the morphological structure of the filaments between circular filaments and dog-bone filaments (formed due to rapid and incomplete solvent extraction); and or some voids in the filaments. All these defects will affect the physical properties of carbon fibers ultimately obtained. The solvent extraction stage is followed with further washing and drying of the filaments.
Schematic of PAN precursor filament wet spinning process

Alternative to the wet spinning process described above, PAN fibers can also be spun using a dry spinning process wherein the solvent extraction stage takes place in a column which is at a temperature above the boiling point of the solvent. This process requires a solvent, which is non-toxic, readily dissolves the polymer, have a low boiling point, and has a low risk of explosion. There are other alternatives techniques to spinning precursor filaments such as air gap spinning, and melt spinning which have also yielded precursor filaments with required properties to be processed into carbon fibers. Each of these process technologies also have other allied processing stages such as stretching (improves the orientation of molecular chains); and chemical treatment (improves handling).
Oxidation / Stabilization The precursor fibers are stabilized by controlled low temperature heating (150 – 300°C) in air to convert the precursor to a form that can be further heat-treated. The fibers are stabilized to preserve the molecular structure generated by the spun fibers. The optimal heating rate would depend upon the chemical composition of the filaments and it’s diameter, as they would determine the stabilization reaction rates. The heating rates vary from isothermal to step-wise (hyperbolic rates) and have different consequences. However, irrespective of the heating rate it should be borne in mind that PAN is a poor conductor of heat and that a run-away exotherm is to be avoided during the stabilization reaction. In the oxidation stage, the PAN fiber will increase in density from 1.18 g/cc to about 1.36 – 1.38 g/cc for the oxidized pan fiber

Oxidative stabilization is performed by passing the fibers through a series of air heated multi-zone ovens with gradually increasing temperatures. The hot air (~250°C) 1. heats the fiber and provides oxygen (O2) for the stabilization reaction, besides removing exhaust components and exothermic reaction heat from the fiber. Significant increases in line speeds are achieved by using more oven in series to provide additional zones. The gaseous by-products of the stabilization reaction include HCN, H2O, CO2, CO, NH3 and it’s important that these gases be displaced continuously (usually to a thermal incinerator) to maintain a safe working environment.	Schematic of stabilization reaction

Carbonization Carbonization of oxidized pan fibers is achieved in a two-stage process usually referred to as Low temperature carbonization and High temperature carbonization. Low temperature carbonization is performed in a Harper LT furnace which is a multizone electrically heated slot furnace which is purged at both ends with Nitrogen (N2) to prevent the ingress of oxygen into the furnace and also helps the removal of evolved tar and other gases. The temperature in the Harper LT furnace is gradually increased in each zone from 400°C to 900°C. High temperature carbonization is performed in a similar Harper HT furnace which is a slot furnace with nitrogen purges at either ends of the furnace to prevent the ingress of air into the furnace and temperatures of the furnaces are increased gradually zone wise from 1000°C to 1800°C. It is known that with increasing carbonization temperature the modulus and tensile strength increase Sometimes, ultra-high temperature carbonization is also performed in Harper UHT furnaces which are specifically designed to heat the fibers to a temperature of about 2500°C and more. With the increase in carbonization temperature, the graphitic content of the fibers (sp2 carbon) increases thereby increasing the modulus and the transport properties. The Carbon fibers obtained from the carbonization process are then surface treated in an electrolytic bath; then sized and collected using online winders. Harper has the experience to help you install a full carbon fiber line. Interesting Industrial Misconceptions • Carbon in PAN based carbon fibers exists as disordered carbon (mix of sp2 and sp3 carbons) and contains very little graphite if any. In this sense, it is a misconception that PAN carbon fibers are graphitized as the carbon in PAN exists in turbostratic form that can only be “carbonized”. Only pitch based fibers are truly graphitic fibers. • Dry spinning process produces fibers as good as the wet spinning process and are fundamentally similar.

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