Modeling Study Of Preparing Spinning Dope For Polyacrylonitrile Based Carbon Fiber By Solution Copolymerization

Free radical copolymerization of ancrylontrile （AN） and itaconic acid （IA） in dimethylsulfoxide （DMSO） initiated by azobisisobutyronitrile （AIBN） is a traditional process for industrial production of precursor materials for polyacrylonitrile （PAN） based carbon fiber. This old process characterizes by long reaction time, high exothermicity, volume shrinkage and viscosity explosion. In contrast to the contributions concerned with developing new synthetic methods, such as mixed solvent polymerization, aqueous suspension or precipitation polymerization etc., or introducing new comonomers and inititators, the copolymerization kinetics and physical properties of polymer solution were detailed studied according to the conventional process in this dissertation from the view point of reaction engineering. A new type reactor with large length-diamiter ratio, high mass and heat transfer efficiency, as well as large scale up potentiality was proposed and expected to operate stably in plug flow model. Specifically, the main results obtained in this paper were listed as follows:（1） The reactivity ratios of AN and IA, and their temperature dependence were reinvestigaed by element analysis method. It was confirmed that the reactivity ratio of AN was in the range of0.30-0.45from50to80℃, while that of IA was still ambiguous because of its "self-retardation" effect in the polymerization. Nitrogen evolution technique was utilized to determine the decomposition rate constant of AIBN in DMSO, and it could be expressed as kd=4.3375×1015exp（-131054/RT） s-1.The lumped reaction rate constant of homopolymerization of AN, kp/kt0.5, measured from low conversion data agreed with the reported values.（2） Based on the conversion-time data of homopolymerization of AN, we found that the primary radical initiation was highly diffusion-controlled beyond moderate coversion, although the radical chain termination and propagation were still seem to be diffusion-independent for the systems with initial monomer concentration of less than4.5mol/L. The initial value of initiator efficiency （f） was properly taken as0.5after comparing some reported values. The experimental results showed that f was changed silightly up to an advanced stage in the polymerization （about60%conversion）, and then falled quite abruptly. A free volume based semi-emprical equation,f=0.5exp（-0.4215（1/VF-1/0.1104））, was derived to describe the variation of f with conversion.（3） The effects of copolymerization rate and molecular weight averages behaviour on the polymerization conditions were studied comprehensively by experimental and modeling approaches. The copolymerization was carried out in a10-liter new designed batch horizontal loop reactor. It was observed that both the chemical-controlled terminal and penultimate models were competent to predict the copolymerization rate feature up to4.5mol/L initial monomer concentration. For system with higher initial monomer concentration, a semi-emprical expression, kt=kt0（Mw0/Mw）1.75exp（-0.17（1/VF-1/（11.9VF0-1.38）））, also based on the free volume theory was used to characterize the gel-effect. It was proved that the mixing intensity and chain transfer reactions had negligible effect on the overall copolymerization rate. It was also found that the polydispersity index of PAN almost maintained at round2.0for system with common initial monomer concentration, and increased materially at the end of polymerization. Ultral-high molecular weight PAN was completely possible to be prepared by solution polymerization under elaborate conditions, i.e., high initial monomer concentration, low initiator feed and low temperature. It was shown that, reaction temperature was the most flexible variable to regulate and control the molecular weight of PAN, and the simulation results suggested that a suitable temperature program may obtain high performance spinning dope without excessively broad molecular weight distribution.（4） The zero-shear viscosity histories with monomer conversion were also determined by a programmable torque rheometer. The Arrhenius-Frenkel-Eyring equation was extended to identify the dependence of zero-shear viscosity on polymer content, temperature and viscosity-average molecular weight of PAN. It was observed that small differences in feed composition and reaction temperature would give rise to a large variation on the zero-shear viscosity. The viscosity increased quickly from about2mPa-s to2-3Pa-s in the early stage of polymerization, and a small increase of polymer content could give rise to a great increase in viscosity when polymer content reached some critical values （about₁0%）.（5） Based on the results of polymerization experiment and model simulation, a parallel dual-axial horizontal tube-like stirred reactor model was designed and manufactured, which had the advantages of large specific heat exchange area. A new on-line determination method based on photo-electric conversion technique was also proposed to investigate the residence time distribution of high viscosity fluid in this type of reactor. It is shown that the double helical mixer had high mixing efficiency, but was hard to get a plug flow pattern.