【作者】 钟磊；

【导师】 梁基照；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2010， 博士

【摘要】 拉伸流动广泛的存在于聚合物材料加工过程中,熔体的拉伸流动行为将对聚合物材料的加工工艺过程和制品的最终力学性能产生重要的影响。深入了解各种聚合物材料体系的拉伸流变性能,能够有效的优化生产过程和提高产品质量。本文从理论建模、数值模拟及实验测量三个方面全面系统的研究了聚乙烯熔体在拉伸流动过程中的行为及其机理。构建了四个聚合物熔体的拉伸黏度模型,其中,基于White-Metzner模型的拉伸黏度模型可以合理的描述熔体的剪切黏度和拉伸黏度的关系;基于Moore动力学方程的黏度模型改进了原有的动力学方程,能够准确的描述聚合物熔体的剪切流变和拉伸流变行为;引入了温度参数的PTT模型比较清晰的体现了拉伸黏度与温度之间的关系;基于Cross方程的经验模型形式简洁,适合应用于有限元模拟。在本文中,不仅利用实验数据对这四个模型进行了验证,还对模型中各个参数的作用进行了分析。利用MATLAB对入口收敛流动分析中的Cogswell模型,Binding模型和Liang模型进行了数值模拟,对三个模型中影响入口收敛流动自然收敛角、边界流线以及涡流区长度的各个参数进行了深入的分析和比较。在FLUENT上利用基于Cross方程的拉伸黏度模型对典型的流道收缩比为4:1的入口收敛流动进行了有限元模拟,分析了由于参数变化而造成的特劳顿比(Trouton ratio)的变化对于形成的收敛流道涡流区大小的影响。用熔融纺丝法测量了低密度聚乙烯(LDPE)、线性低密度聚乙烯(LLDPE)、高密度聚乙烯(HDPE)三种聚乙烯和聚丙烯(PP)熔体的拉伸流变性能。分析了温度对于材料的熔体强度和可拉伸性的影响,进一步利用Arrhenius方程计算了四种材料的熔体强度活化能,比较了四种材料的温度敏感性。利用“局部方法”的计算结果,绘制了拉伸应力与拉伸应变速率,拉伸黏度与拉伸应变速率的关系曲线,研究了拉伸应变速率、温度和挤出速度对于拉伸应力和拉伸黏度的影响。根据熔融纺丝测量结果,绘制了材料的拉伸流变主曲线。利用主曲线对LDPE和LLDPE的拉伸流变性能进行了比较,深入的分析了比例因子b与温度和挤出速度的关系,根据b与不同温度和挤出速度下熔体拉伸曲线的对应关系,对设定条件下的熔体的拉伸黏度进行了估算,将结果与测量数据进行了比较。基于Cogswell模型,用入口收敛法测量了LDPE和LLDPE的拉伸黏度,并与熔融纺丝测量结果进行了比较。更多还原

【Abstract】 In many processing operations, polymer melts experience extensional flows. The elongation behaviors of polymer melts have important effects on the processing procedures and the mechanical properties of final products. The thorough knowledge of the extensional properties of polymer can effectively optimize the production procedure and improve the products quality. In this article, we comprehensively studied the extensional flow behaviors and mechanisms of polyethylene melts from the modeling, simulation and measurements.Four extensional viscosity models were derived in this work The model based on White-Metzner equation is reasonable to describe the relation between shear viscosity and extensional viscosity; the model based on Moore dynamic equation that with some modifications of the original Moore equation can correctly describe both the shear and extensional rheology of polymer melts; the PTT model with temperature parameter can clearly demonstrate the relation between temperature and extensional viscosity; the model based on Cross model is simple and suitable to apply in finite element simulation. These four models were verified with experiment results and the effects of their parameters were analyzedNumerical simulation of Cogswell model, Binding model and Liang model were performed on MATLAB. The effects of model parameters on the natural convergent angle, boundary streamline and vortex length were discussed. Finite element simulations of 4:1 entrance flow were performed on the FLUENT with the extensional viscosity model based on Cross model. The effects of the change of Trouton ratio on the formation of vortex size were discussed.The extensional rheological properties of low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE) and polypropylene (PP) were measured by melt spinning technique. The influences of temperature on the melt strength and draw ability were analyzed. Applied the Arrhenius equation to calculate the activation energy of melt strength for these four materials and compared the temperature sensitivity of them. A“Local match”was used to obtain the extensional stress and viscosity. The curves of extensional stress versus strain rate and extensional viscosity versus strain rate were plotted by the results. The effects of extensional strain rate, temperature and extrusion velocity on the extensional stress and viscosity were discussed. According to the measurement curves, the mastercurves of melt spinning were drawn. The properties of LDPE and LLDPE were compared by their mastercurves. The relations between the ratio b with temperature and with extrusion velocity were comprehensively investigated. According to result that each b had a unique relation with temperature and with extrusion velocity, the extensional viscosities of some setting conditions were calculated. The results of estimation were compared with experimental data. The entrance flow measurements that based on Cogswell model were performed to obtain the extensional viscosity of LDPE and LLDPE and the results were compared with those obtained by melt spinning technique.更多还原