Thermal polyacrylonitrile

No.14, Oct.l998,p.2503-12 SPECTROSCOPIC INVESTIGATIONS OF POLYACRYLONITRILE THERMAL DEGRADATION... 

Polymer Solvent. Sulfolane is a solvent for a variety of polymers, including polyacrylonitrile (PAN), poly(vinyhdene cyanide), poly(vinyl chloride) (PVC), poly(vinyl fluoride), and polysulfones (124—129). Sulfolane solutions of PAN, poly(vinyhdene cyanide), and PVC have been patented for fiber-spinning processes, in which the relatively low solution viscosity, good thermal stabiUty, and comparatively low solvent toxicity of sulfolane are advantageous. Powdered perfluorocarbon copolymers bearing sulfo or carboxy groups have been prepared by precipitation from sulfolane solution with toluene at temperatures below 300°C. Particle sizes of 0.5—100 p.m result. 

More than 95% of current carbon fiber production for advanced composite appHcations is based on the thermal conversion of polyacrylonitrile (PAN) or pitch precursors to carbon or graphite fibers. Generally, the conversion of PAN or pitch precursor to carbon fiber involves similar process steps fiber formation, ie, spinning, stabilization to thermoset the fiber, carbonization—graphitization, surface treatment, and sizing. Schematic process flow diagrams are shown in Eigure 4. However, specific process details differ. 

Many polymer films, eg, polyethylene and polyacrylonitrile, are permeable to carbon tetrachloride vapor (1). Carbon tetrachloride vapor affects the explosion limits of several gaseous mixtures, eg, air-hydrogen and air-methane. The extinctive effect that carbon tetrachloride has on a flame, mainly because of its cooling action, is derived from its high thermal capacity (2). 

Bead Polymerization Bulk reaction proceeds in independent droplets of 10 to 1,000 [Lm diameter suspended in water or other medium and insulated from each other by some colloid. A typical suspending agent is polyvinyl alcohol dissolved in water. The polymerization can be done to high conversion. Temperature control is easy because of the moderating thermal effect of the water and its low viscosity. The suspensions sometimes are unstable and agitation may be critical. Only batch reaciors appear to be in industrial use polyvinyl acetate in methanol, copolymers of acrylates and methacrylates, polyacrylonitrile in aqueous ZnCh solution, and others. Bead polymerization of styrene takes 8 to 12 h. 

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

Because of their unique blend of properties, composites reinforced with high performance carbon fibers find use in many structural applications. However, it is possible to produce carbon fibers with very different properties, depending on the precursor used and processing conditions employed. Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers, polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market (and represents about 90% of the total carbon fiber production), while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. This chapter compares the processes, structures, and properties of these two classes of fibers. 

Oxide-water interfaces, in silica polymer-metal ion solutions, 22 460—461 Oxidimetric method, 25 145 Oxidization devices, 10 77-96 catalytic oxidization, 10 78—96 thermal oxidation, 20 77-78 Oxidized mercury, 23 181 Oxidized polyacrylonitrile fiber (OPF), 23 384... 

Wet spinning. This technique is characterized by spinning a filtered viscous polymer mass, dissolved in a suitable solvent, into contact with a precipitation or coagulation bath. Polyacrylonitrile, polyvinyl acetate, cellulose acetate, and other materials are processed by this method. Thermal requirements for pigments are less stringent than for melt spinning but pigments are expected to be fast to the solvents and chemicals used. 

P.B.15 3, like stabilized a-Copper Phthalocyanine Blue, markedly affects the hardening of unsaturated polyester cast resins. The list of applications also includes PUR foam materials, office articles, such as colored pencils, wax crayons, and water colors, as well as spin dyeing of polypropylene, polyacrylonitrile, secondary acetate, polyamide, polyester, and viscose. Used in polyester spin dyeing, P.B.15 3 satisfies the thermal requirements of the condensation process (Sec. 1.8.3.8). 1/3 and 1/25 SD samples equal step 7-8 on the Blue Scale for lightfastness. Textile fastnesses, such as stability to wet and dry crocking are perfect. 

The ruthenium(II) polypyridyl complexes are also popular but the brightnesses do not exceed 15,000 and thermal quenching is rather significant. This property can be utilized to design temperature-sensitive probes providing that the dyes are effectively shielded from oxygen (e.g., in polyacrylonitrile beads). Despite often very high emission quantum yields the visible absorption of cyclometallated complexes of iridium(III) and platinum(II) is usually poor (e < 10,000 M-1cm-1), thus,... 

Cyclization is a key reaction in the production of carbon fibers from polyacrylonitrile (PAN) (acrylic fiber see Sec. 3-14d-2). The acrylic fiber used for this purpose usually contains no more than 0.5-5% comonomer (usually methyl acrylate or methacrylate or methacrylic acid). Highly drawn (oriented) fibers are subjected to successive thermal treatments—initially 200-300°C in air followed by 1200-2000°C in nitrogen [Riggs, 1985]. PAN undergoes cyclization via polymerization through the nitrile groups to form a ladder structure (XXVII). Further reaction results in aromatization to the polyquinizarine structure (XXVIII)... 

Zipping-up reaction is a well known method for ladder polymer production. The most important from the practical point of view is cyclization and carbonization of polyacrylonitrile. " The mechanism of the thermal conversion of polyacrylonitrile to carbonized product has been intensively studied and it is extremely complex.The reaction can be partially treated as a template polymerization proceeding according to the reaction ... 

The details of so-called Black Orion production have been given in many papers. Also, the properties of carbonized polyacrylonitrile are well known because of which it is in demand especially as a component of composites, mainly because of its strength and high thermal resistance. 

Within polymer solids, volatile, reactive fragments are trapped and often rereact, forming rearranged structures. If the rearranged structures exhibit markedly better stability, excessive char results. Thus solid ldpe decomposes with little char, whereas polyacrylonitrile (PAN) gives excessive char because of the formation of thermally stable rearranged products. 

Thermal treatment of polyacrylonitrile at 200-300 °C leads to the appearance of conjugated bonds and electronic semiconductor properties. The main photoconductivity maximum is situated at 420 nm. An increase of the temperature treatment shifts the photosensitivity to the long wavelengths. The estimated mobilities were from 10" 7 to 104m2 V"1 s"1. The main results obtained proved the sufficiency of the conjugated bonds for the appearance of the semiconductive properties. 

A series of papers have rqwrted FT-IR studies of the thermal degradation of polyacrylonitrile 22S-226,227 228, 229, 230>. These FT-IR measurements indicate that the nitrile groups play an important role in the degradation process even in the initial stages and show a steady decrease in nitrile with time both in air and under reduced pressure. These results are contrary to previous dispersive infrared measurements... 

Polyacrylonitrile has been the subject of investigations by Russian workers, particularly those aspects relating to its thermal degradation. Polyacrylonitrile itself is a semiconductor with an activation energy of 0.32 to 0.84 e. v. (69) or 1.5 to 4.6 e. v. (70) depending upon the method of polymerization, ambient atmosphere and sample history. 

Ceramic fibers used in composites are usually made by high-temperature methods. Carbon (graphite) fiber, for example, can be made by the thermal decomposition of fibers of polyacrylonitrile, a long-chain organic molecule also used to make the textile Orion ... 

It is also possible to prepare all-carbon polymers of closely related structure. For example, pyrolysis of polyacrylonitrile, (-CH2CHCN-)X, first results in cyclization of some of the -CN side chains.61 Prolonged pyrolysis yields very pure graphitic material. It is very strong and has high thermal stability. In the form of fibers, it can be used for reinforcement in high-performance composites. Additional information on pyrolysis is given in Chapter 9. 

Other fibers. The other major class of synthetic fibers, the polyacrylonitriles (orlon, acrilon, etc.) like the cellulosics (rayon, cotton) show no thermal activity up to 300 °C. Above these temperatures degradation of sample accompanies any characteristic transitions or curing exotherms. To minimize this effect, the samples are run in an inert environment such as N2, as seen in figure 16. Under these conditions reproducible characteristic endotherms were obtained for identifying wool, cotton and rayon. In roughly the same temperature region,... 

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