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Oxidative thermal stabilization of acrylic twists has a relatively high rate at a temperature close to the temperature of autocatalytic heating (250-260~To Oxidative thermal stabilization is one of the most important process stages in production of high-strength carbon fibres. It is conducted with the twists under tension in air medium at a temperature of 180-300~ for 1-3 h. In comparison to carbonization, which lasts for 5-10 min, thermal stabilization is a slow stage which makes it difficult to group the oxidation and carbonization furnaces in one highly efficient production line. To increase the productivity of the equipment, the thermal stabilization process is conducted at the maximum possible temperature, where the slightest overheating will cause autocatalytic spontaneous combustion of the material. For this reason, correctly selecting the method of tempering acrylic twists during their heat treatment is very important.In oxidative thermal stabilization of potyacrylonitrile or copolymers of acrylonitrile at the indicated temperatures, in addition to oxidative processes accompanied by addition of 8-12% oxygen, cross-linking of the macromolecules by formation of intermolecular cross links, partial pyrolysis of the polymer with liberation of acrylonitrile, ammonia, and hydrogen cyanide, and polymerization of nitrile groups with formation of cyclic ladder oligomers take place [1]. Some of these processes take place with release of a large amount of heat, characterized by steep elevation of the exothermic peak on the differential scanning calorimetry (DSC) curve. The beginning of the peak is observed at 180-210~ as a function of the composition of the copolymer, supermolecular structure of the fibre, and heating rate, and the peak maximum falls in the region of 260-290~ The exothermic effect is 70-150 cal/g. At the high heating rate of 2-15~ the heat flux can attain 16-160 cal/(g.sec), which can result in autocatalytie overheating and destruction of the fibre with insufficient heat transfer.The situation is complicated by the fact that to guarantee high efficiency of the equipment, the process must be conducted at the highest possible temperature (240-260~ which is close to the maximum of the exothermic effect and makes the process difficult to control. The reaction of polymerization of CN groups probably makes the greatest contribution to the exothermic effect. The kinetics of this reaction is usually characterized by the decrease in the intensity of the 2240 cm -1 absorption band corresponding to stretching vibrations of CN groups. The high activation energy of the polymerization reaction, 43 kcal/mole [2], indicates its important dependence on the temperature. The reaction rate constant increases sharply beginning at 220~ -All-Russian Scientific-Research Institute of Polymer Fibres, Mytishchi.
It was found that in thelwlooxidation of PAN fibres in air byDuring thermal oxidation, polyacrylonitrile (PAN) fibres become thermally stable, which makes high-temperature treatment, i.e., carbonization, possible. Thermooxidation and carbonization are most important in the production of carbon fibres.PAN fibres in the form of strips are oxidized in passing through a multipass furnace fed air heated to 180-270~ In this case, there is induced convective heat transfer between the hot air and the PAN fibre, characterized by the convective heat transfer coefficient. This type of heat transfer takes place comparatively slowly, which also detemfines the greater duration of thennooxidation.The stage of thennooxidation can be intensified by using another type of heat transfer --conductive (contact), where heat is transferred due to the themaal conductivity of the contacting materials characterized by a high heat transfer coefficient. The duration of thermooxidation in this case is several times shorter than with the convective method. Setting a sufficient oxidation time for PAN fibres by the contact method is one of the basic problems of the research described in the present article.Oxidation of PAN fibre is an exothennic process associated with heat release of 110-150 cal/g of fibre. In view of the low heat capacity of PAN fibres, 0.36 cal/(g.K), with a low rate of elimination of heat as in convective heat transfer, the exothemlic heat can heat the fibre above the critical values (285-290~ at which autocatalytic decomposition of the polymer begins. For this reason, in furnaces with convective tempering, the process is conducted 30-35~ below the critical temperature, i.e., at 225-255~ which reduces the rate and increases the duration to 2.5-3.5 h.In contact tempering, which provides for close contact of the PAN fibres and surface of the material with high thermal conductivity and heat capacity, this surface simultaneously performs the function of elimination of heat from the heat carrier to the fibre and elimination of exothemaic heat, i.e., it plays the role of a thermostat. For this reason, the interval between the temperature of the process and the critical temperature of the onset of autocatalytic decomposition of the polymer can be significantly reduced with no risk of spontaneous ignition, and the process can be conducted in the region of higher temperatures, 260-280~ which in turn reduces the duration of oxidation.When PAN fibres are heated in air, a series of chemical reactions takes place --cyclization, dehydrogenation, oxidation, pyrolysis, intra-(or inter-) molecular cross-linking [ 1 ]. Each of these reactions is characterized by its own rate and activation energy, which determines its temperature curve. In conditions of rapid contact oxidation, the ratio of the rates of the above reactions can differ from the ratio characteristic of slow thermooxidation.We attempted to comparatively study contact and convective thermooxidation. The studies were conducted on a laboratory setup with electric heating (Fig. 1). P...
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