Acrylonitrile And Vinyl Acetate Aqueous Precipitation For Copolymerization

Acrylic fiber is acylonitrile copolymer fiber with acylonitrile fraction more than 85wt%. It is welcomed very much due to the wool-like clothing properties compared to other synthetic fibers and has become one of the important raw materials in the world textile and other light industries. A second comonomer, such as methacrylate (MA), vinyl acetate (VAc), itaconic acid (IA), and acrylic acid (AA), is usually incorporated into acrylic fiber to modify the spinning properties of fiber. Up to now, methacrylate (MA) and vinyl acetate (VAc) are most used as second comonomers in domestic acrylic fiber industry. The resource of VAc raw material is abundant with low price, and the fiber with high VAc content can also perform the high retractility. Therefore, acrylic fiber with high VAc content has received a lot of attention.In this work, the aqueous precipitation copolymerization of AN and VAc was investigated in a continuous pilot apparatus, which was scaled down from an industrial process. The experiments were carried out with different initiation systems and various pH values of polymerization system. It had been found that that the on-line monitor and control system of the polymerization temperature and pH could ensure stability and replicability of the polymerization process. For start-up with polymer seed, polymerization temperature and pH generally reach their stable state in half and two residence times respectively while the monomer conversion and sedimentation value became stable after three residence times. The comparison between pilot experimental and industrial process data has proven that the continuous pilot plant could repeat the industrial process. Meanwhile, two common unstable phenomena in AN aqueous precipitation polymerization had been analyzed. When the pH value of polymerization system was increased over 3.0, run away polymerization would take place very easily. On the other hand, when the pH value was further increased over 4.0, deactivation phenomenon would occur.VAc content in Copolymers had been successfully increased by increasing VAc content in monomers feeding and pH value of polymerization system. The conversion was increased slightly correspondingly, which meet requirements for high VAc content acrylic fiber. However, the average molecular weight of copolymer was increased apparently, which contradict the results of theoretical analysis. Further investigation is necessary to reveal the reasons of this phenomenon. Molecular weight was the key factor determining the viscosity of spinning dope. If the molecular weight is increased excessively, the viscosity of spinning dope will be increased dramatically, which was not favorable for post-processing and fiber formation. In order to lower the molecular weight of AN copolymer, the amount of chain transfer agent (TEG) and the oxidant in initiation system were separately increased. It was found that with 5% increase in either chain transfer agent or oxidant concentration, polymerization conversion was increased slightly, while VAc composition content in copolymer and viscosity average molecular weight decreased significantly. However, further increase in either chain transfer agent or oxidant concentration did not have significant effects on resultant copolymer. In addition, when the average residence time was shortened from 47 minutes to 44 minutes, polymerization conversion, polymer composition and viscosity average molecular weight were not significantly affected, which might improve production capacity moderately.When VAc content in the copolymer was increased, both volume average and number average particle sizes were increased, particle size distribution became narrow, and sedimentation value was declined. It is worth noting that the increase in number average particle size was greater, which indicated that cohesion rate between small particles was increased. When pH was increased, volume average particle size was increased first then decreased, while number average particle size was decreased first then increased. The distribution index of particle size (PDI) was increased first then decreased, while the coefficient of dispersibility (C.V.) was decreased and sedimentation value did not change significantly. When average residence time was increased, number average particle size was not changed, volume average particle size was increased, particle size distribution became broader slightly, and the sedimentation value was declined slightly. When the amount of chain transfer agent addition or oxidant was increased, both volume average and number average particle sizes were decreased, particle size distribution was broadened, the number of small particles was increased, and sedimentation value was increased slightly.Rheological properties of acrylonitrile copolymer in NaSCN-aqueous solution have been investigated. It was found that the shear stress and shear rate showed a linear relationship in the double logarithmic coordinates, which meet with power-law. The solution is a shear thinning non-Newtonian fluid, also called pseudoplastic fluid. The shear viscosities of solution with similar Copolymers composition was increased with the increase of the molecular weight. The non-Newtonian index n was not obviously changed, while notable increase of apparent viscosityη_αand consistency coefficient k_sp have been found with the increase of copolymer molecular weight. The shear viscosities of solution of Copolymers with similar molecular weight were increased with the increase of the VAc content in copolymer. Consistency coefficient k_sp was decreased significantly, while non-Newtonian index n was not changed.