The Catalytic Mechanism Of Carbon Fibers From Viscose Rayon Precursor In Low Temperature By Sulfuric Acid-Urea Pretreatment

Rayon fiber is one of important raw materials for the manufacture of carbon fibers because of its availability,low cost and non-melting character.Rayon based carbon fibers have low thermal conductivity,low density and high breaking elongation characteristics,which allow them to be used in the aerospace industry as structural reinforcement for phenolic resin based ablative materials.The preparation of rayon based carbon fibers was carried out in two main stages, pyrolysis and carbonization.According to literatures,pyrolysis of rayon fibers typically leads to a solid residue(char),high boiling volatiles(tar) and gaseous products.These products are formed by two competitive pathways.The first pathway involves dehydration,rearrangement,formation of carbonyl groups,evolution of carbon monoxide and carbon dioxides,and formation of a carbonaceous residue. These reactions are accelerated in the presence of a variety of organic and inorganic catalysts,particularly Lewis acids,which catalyze the dehydration reactions.In the second pathway,which competes with the first pathway,the thermal scission of glycosidic bonds between the glucopyranose units of the cellulose produces many oxygenated compounds.These oxygenated compounds account for the major weight loss of the solid residue.Therefore,control of reaction pathways in the initial stage of pyrolysis is important for preparing high quality rayon based carbon fibers.Since catalysts accelerate the pathway of the pyrolysis reactions,it is essential to study the effects of catalysts on the pyrolysis of rayon fibers.An effective catalyst should have the following functions.First,the catalyst should decrease the pyrolysis temperature to a lower range.Second,it should be able to increase the amount of water and carbon dioxide produced in the reaction.Finally,it should increase the yield of char. The catalysts are of importance for the production of carbon fiber.The mixture of sulfuric acid and urea as composite flame retardant is an effective catalytic system proved by experiments in the research.First of all,the adhesion of catalyst was discussed and the calculation method of adhesion ratio was established.In the presence of catalyst,the yield of residue and mechanical properties become higher.And there is closely relationship between the catalyst and properties through analysis of the tests of related performances such as breaking strength,crystallinity,orientation and degree of polymerization,carbon content and so on.The low temperature pyrolysis process and catalytic mechanism of cellulose impregnated with sulfuric acid-urea pretreatment were mainly studied in the paper. The thermal degradation of cellulose and its solid and gaseous products were investigated by TGA,DSC and IR analysises respectively.Significant conclutions are obtained as follows:(1)The onset of dehydration and maximum rate of the loss of mass are shifted to the region of low temperature.The dehydration reactions of cellulose are moved forward and more complete.(2)The pyrolysis process of cellulose in the present of sulfuric acid are carried out in three steps.The process can close hydroxyl groups and inhibit the formation of levoglucosan,therefore it is favorable for improving the yield of residue and mechanical properties,and shorten the time of carbonization.(3)The sulfuric acid is primary in the catalyst system,and the urea can relax the acute catalysis of sulfuric acid.DMA is used to analysis the thermal shringkage of viscose fibres during low temperature heat-treatment.The affecting factors including catalyst,preloaded tension, etc.were investigated.More information is provided for production process of RCF.