Intensification of thermooxidation of polyacrylonitrile fibres

Abstract

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...

Improvement of Manufacturing Technology for Ultrastrong and High-Modulus Carbon Fibres

“…Its duration reaches 3 h at 180-300°C, which inevitably results in large heat losses. They can be eliminated by going from convective to conductive (contact) tempering [9,10].…”